I don't find that argument particularly compelling, since it would not be shocking if the readings shifted to the vernacular earlier than a full translation of the Liturgy. The story generally goes that St. Cyril translated St. Mark's Liturgy, and that's why it bears his name today. So I would need stronger evidence than a Coptic speaking saint understanding the Gospel to question that. Though of course I would be surprised if Coptic supplanted Greek immediately after the translation was undertaken with no period of mixed usage, but the question from the OP was the time that Coptic began to be used as the language of the Liturgy.
Full translation? That didn't happen until it was translated into Arabic.