The Muslims who invaded Egypt and enslaved Egypt's Coptic population did not write in Greek or in Coptic.
They were Arabs, and the ones who could write wrote in Arabic.
There are no Muslim texts from the period in either Greek or Coptic.
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.