If we read what Al Bayhaqi says in regards to the saying he was upon the deen of his people we know it does not mean that The Prophet was a mushrik.

I saw the Messenger of Allah ﷺ while he was upon the religion of his people, and he was standing on his camel at Arafat among his people until he departed with them. This was a success from Allah for him.
Al-Bayhaqi said: the meaning of his words “upon the religion of his people” is what remained from the heritage of Ibrahim and Ismail peace be upon them, and he never committed shirk with Allah at all.
Albani says from this statement also, we understand that he used to stand at Arafat even before revelation came to him. This was a success from Allah for him
Sahih as-Sirah an-Nabawiyyah By Shaykh Al-Albani Page 33
First I will just quote Ibn Taymiyyah mentioning the ikhtilaaf on if prophets can sin before revelation as that is the context of the passage that Rob Christian quotes, then I will show Imam Ibn Qutaybah (d. 276AH) explaining the hadith, to mean that the prophet was upon the religion of Ibrahim عليه السلام not that he was upon the religion of the mushrikeen.

And the point is that this disagreement about the ability to sins by the Prophets before Prophethood is not limited to the view of the Muʿtazilah alone, rather it also exists among the people of hadith and Ahl al-Sunnah. (The “people of hadith” are the scholars of the Hijaz, such as the companions of Malik ibn Anas, the companions of Muhammad ibn Idris al-Shafiʿi, the companions of Sufyan al-Thawri, the companions of Ahmad ibn Hanbal, and the companions of Dawud ibn Ali al-Asfahani. They were called the “people of hadith” because of their devotion to preserving hadith, transmitting reports, and basing rulings on textual evidence. They would not resort to clear or subtle analogical reasoning so long as they found a report or narration. See: al-Milal wa al-Nihal by al-Shahrastani, vol. 2, p. 38, and al-Mawsuʿah al-ʿArabiyyah al-Muyassarah, vol. 1, p. 252)
Abu Bakr ibn al-Tayyib said: And many of them said, among our own companions and the people of truth: It is not impossible that someone who was a disbeliever or who committed major sins before being sent as a Prophet could be chosen for Prophethood. He said: There is nothing with us that prevents that, and we will explain the reasoning on that. (He refers here to some of the Muʿtazilah. This was the view of Abu Hudhayl and Abu Ali al-Jubba’i. Fakhr al-Din al-Razi mentioned this in his book ʿIsmat al-Anbiyaʾ, p. 27, saying: “This is the view of most of our companions.”)
And they differed regarding the occurrence of sins from them after being sent with the message.
The Rafidah and those who followed them said: It is not permissible for them to commit any sin, whether minor or major. Nor is it permissible for them to forget or make mistakes in their conveyance of the message, nor in anything else. (The Rafidah are a branch of the Shia, consisting of several groups. They were called “Rafidah” because they rejected the leadership of Abu Bakr and ʿUmar. They unanimously believe that the Prophet ﷺ explicitly appointed Ali ibn Abi Talib by name as his successor and declared it publicly, and that most of the Sahabah went astray by not following him after the Prophet’s death… and other such falsehoods.)
The Muʿtazilah said: Minor sins may occur from them during their mission, even while being aware of the danger and ugliness of such acts. However, it is not permissible that major sins occur from them, nor the minor sins that are deemed disgraceful and that lower the status of the one who commits them.
And a group among them said: It is not permissible for sins to occur from them knowingly and intentionally, while being aware of their ugliness and that they are forbidden. Rather, what may occur from them happens by way of misjudgement in interpretation. This is the view of al-Jubba’i and many of their salaf. (He is Abu Ali Muhammad ibn Abdul Wahhab al-Basri al-Jubba’i, the leader of the Muʿtazilah and author of many works. Despite his innovation, he had wide knowledge and sharp intellect. He was the one who made kalam easier and more accessible. Among his works are: al-Tafsir al-Kabir, Mutashabih al-Quran, and al-Ijtihad, among many others. Al-Jubba’i is attributed to a village near Basra called Jubba. He died in the year 303 AH. See his biography in: al-Lubab fi Tahdhib al-Ansab, vol. 1, p. 255, Wafayat al-Aʿyan, vol. 4, p. 267–269, biography no. 608, Siyar Aʿlam al-Nubalaʾ, vol. 14, p. 183–184, biography no. 102, Tabaqat al-Mufassirin by al-Dawudi, vol. 2, p. 191–192, biography no. 529, Shadharāt al-Dhahab, vol. 2, p. 241)
Tafsir Ayāt Ashkalat Ala Kathir Min al-ʿUlamāʾ By Ibn Taymiyyah Page 185-187
An Early Imam, Ibn Qutaybah Says The Hadith لقد رأيت رسول الله ﷺ وهو على دين قومه Means The Prophet Was Upon The Religion Of Ibrahim عليه السلام

Ibn Qutaybah said: And the Messenger of Allah ﷺ was upon the religion of his people, meaning he followed what they were upon of belief in Allah, and he acted according to what remained of their practices such as circumcision, washing, pilgrimage, and belief in resurrection, the Last Day, and recompense. Along with that, he never came near the idols, rather he used to criticise them and said, “They became hated to me.” However, he did not yet know the obligations of Allah and the rulings that Allah prescribed for His servants through him, until revelation was sent to him.
Likewise, Allah the Exalted said: “Did He not find you an orphan and shelter you, and find you lost then guide you” (al-Duhaa: 6–7).
(He means: ‘lost’ in the sense of lacking the details of iman and Islam and its rulings, so Allah the Exalted guided you.)
And similarly His saying: “You did not know what the Book is nor what iman is” (al-Shura: 52),
(He means: you did not know what the Quran was nor the rulings of iman.)
It does not mean iman as affirmation itself, because his forefathers who died upon shirk knew Allah the Exalted and believed in Him, and they used to perform pilgrimage for His sake, while taking gods besides Him which they claimed would bring them closer to Him. They also upheld justice in what they believed, and warned against oppression and its consequences, and would pledge among themselves not to oppress or wrong anyone.
And ʿAbdul Muttalib said to the king of Abyssinia when he asked him what his need was, he replied: “I only seek my camels.” This amazed the king, as he had expected him to request that he withdraw from attacking the House. So he said: Indeed, this House has a Lord who will protect it, or something similar to that. (ʿAbdul Muttalib, the grandfather of the Messenger of Allah ﷺ from his father’s side, was named ʿAbdul Muttalib ibn Hashim ibn ʿAbd Manaf. He was born in Madinah around 127 years before the Hijrah and raised in Makkah. He was white, tall, patient, brave, eloquent in speech, and sharp in mind. He died in Makkah in the year 45 AH before Hijrah.)
These people affirmed belief in Allah the Exalted and believed in Him, so how could it be said that the pure, noble, purified one ﷺ did not believe in Him before revelation? This is something no one should be unaware of or miss. What Allah the Exalted meant in His saying, “You did not know what the Book is nor what iman is” was the rulings and the detailed practices of iman.
Abu Muhammad (Ibn Qutaybah (d.276AH) said: The meaning of this narration is that he ﷺ was upon the religion of Ibrahim and Ismail عليهم السلام
Ta-weel Mukhtalaf il-Hadeeth – Ibn Qutaybah (276H) Page 178-179
