“We don’t know the source of any manuscript, we only know what a given manuscript says.” James White.
An early manuscript has no more proof of its source than a late manuscript. We only know what the manuscript says. We don’t know whether the source was in the same language, making the manuscript a translation(!) or even if the manuscript itself may have been “an original”, not a copy or translation at all! Think about that for a moment as you appeal to the latest axioms of textual critical theory.