I argued that the Berber and Arab conquerors of Spain weren't Muslims in a modern sense of the word. Islam in it's modern form was arguably only starting to take place at this stage. To the people who they conquerored they would have been seen most likely as Christians, although subscribing to a form deemed heretical than followers of a separate religion.
There are serious historical problems about the sources we have for the early history of Islam. The earliest biography of the Prophet Muhammad was Ibn Ishaq which was mid 8th century, which was included in Ibn Hisham which was written around the turn of the 9th century. Sahih Bukhari which contains sayings attributed to Muhammad was written around the mid 9th century, the histories of Al-Tabari are from the early to mid 9th century as well. Those are the earliest sources and apart from Ibn Ishaq (7th century which we don't have a primary source) they date from the early to mid 9th century, at least 200 years after Muhammad had died.
I mean, I agree that Modern Islam is relatively different from its ancestor in the Rashidun Era but it wasn't like it was unrecognizable. In any case, what I was specifically refuting was this claim you made:
Instead we need to consider the Arab and Berber forces being Christians although subscribing to a Nontrinitarian form.
Archeological evidence (as shown earlier) has proven that the modern Qur'an can be radiocarbon dated to the life of the Prophet, which means that the basic text of Islam was already in its final and modern form by the Rashidun Era (which logically precludes any idea that the early Muslims were Christians - the Qur'an makes quite clear that it sees Christianity as a respected old faith, but no longer the divinely-revealed truth). Of course Christians would initially consider Muslims to be heretical Christians - they didn't quite understand what Islam was at first, so they used the closest analogy they had in their experience - if the Qur'an was already in its final form by then, we definitely know that the earliest Muslims didn't consider
themselves Christians.
Also, your point about Ibn Ishaq being the first collector of Seerah is a commonly-cited misconception: both Ibn Shihab az-Zuhri (born only 50 years after Hijrah) and Yazīd ibn Abī Ḥabīb were much older than Ibn Ishaq and Al-Kurtami might be even older. I do agree, though, that many of the hadith sources commonly touted as being the word of Muhammad are rather unverifiably accurate at best and obviously latter-day fabrications at worst.