Does anyone know if this is a valid argument? It sounds like the copyists would not have known Greek grammar in order to have made such a blunder by replacing the tetragrammaton with Kyrios and neglecting to put in the definite article.
All I do know about this is that the experts, who speak read and write the language, as well as other ancient languages, say that the authors of the gospels were very well educated, Hellenised Jews. In fact they were so well educated they even know what prestigious ancient school they are likely to have attended merely due to the way they write and the way they formulate their stories. The stories are layered, like an onion.
In fact I believe it’s Mark people say has slightly ‘rougher’ Greek. Well the experts say that’s not quite true. In fact they say it’s a very stylised Greek, written intentionally that way by a very educated person. Much like, for instance, some authors will write in a colloquial style for entertainment value.
So to answer your question it’s doubtful the author made a grammatical error.
Edit: oh sorry I see you were asking if the later copyists made an error? Sorry I as referring to the original authors and very early copyists.