Both of you are making an argument that’s like saying, “The early Christians in Rome didn’t speak Latin; they spoke Greek.” While a Latin version of the gospel from the 2nd century found in Italy would be big news, it’s fundamentally wrong to suggest that “Christians in Rome” did not speak Latin. ESPECIALLY since the Muslims in 8th-century Egypt would be newcomers, it makes sense to translate something into the local langugae, especially something which is neither Sunnah nor Quran.
The Goths didn't stop to learn Latin while they were pillaging, for example.
It was only later that they acculturated.
The Arabs did not acculturate to Coptic Egypt. They spent centuries imposing Arabic language and religion on the Copts, a process they never fully succeeded in completing.
Greek speakers had been living in Rome for centuries by the time Christianity spread to Rome from Greece and Judaea.
The Romans conquered the Greeks and imported them to Rome.
The Arabs conquered the Copts and sent in settlers from Arabia.
The situation is not to be equated to the situation of Greek-or-Latin speaking Christians in Rome, because the Muslims did not proselytize by translating their religious writings into native languages; rather, they required proselytes to read and recite in Arabic.
This is because of their theological tenet that Arabic is, literally, Allah's invention and his chosen language; that the Koran was literally dictated by him, ipsissisi verbi in words too ineffable to put in any other language.
After Uthman canonized the Koran in 600 whatever, he ordered all variant text to be burned. After that, those producing theological texts in any other version or language would be prosecuted. Even in the early 8th century only a single sura of the Koran had been translated into Persian; it was only in the 10th century that a complete Koran was translated into Persian.
What I'm saying is not only true of the Koran itself; but it reflects to a large extent on all Muslim religious writings. I don't think Christianity ever regarded Aramaic, Hebrew, Greek, or Latin as being the indispensable hieratic languages the way the Muslims regarded Arabic.
So, although I'm no more than guessing, as you are, nevertheless I would incline to the hypothesis that the source here is Coptic-Gnostic rather than Muslim.