About early Muslims and Islam

I have previously asked a question that a Christian asked to me but he is bothering me again,I think he is trying to convince me to leave Islam.But before I block him I think I need to find the answer that he raised,he quoted this"The accounts of non-Muslim conquered peoples also conflict with the accounts of traditional Islamic literature. Examining 7th century Byzantine Christian sources commentary on the Arab "immigrants" (Mhaggraye) who were invading and settling in formerly Byzantine territory at that time, historian Abdul-Massih Saadi found the Christians never mentioned the terms "Quran" nor "Islam" nor that the immigrants were of a new religion.[Note 2] They referred to the immigrants in ethnic terms -- "among them (Arabs) there are many Christians...". The Christians used secular or political, not religious terms (kings, princes, rulers) to refer to the Arab leaders. Muhammad was "the first king of the Mhaggraye", also guide, teacher, leader or great ruler. They did however mention the religion of the Arabs. The immigrants' religion was described as monotheist "in accordance with the Old Law (Old Testament)". When the Emir of the immigrants and Patriarch of the local Christians did have a religious colloquium there was much discussion of the scriptures but no mention of the Quran, "a possible indication that the Quran was not yet in circulation." The Christians reported the Emir was accompanied by "learned Jews", that the immigrants "accepted the Torah just as the Jews and Samaritans", though none of the sources described the immigrants as Jews."Can anyone tell me about the quotation?Is there any possible misunderstanding here?