Irish monks (at least) copied stuff that they knew they disagreed with - pre-Christian pagan stories, for example. See "How the Irish Saved Civilization" for an example.
It's relevant in a one-directional sense. If you have an early 2nd century papyrus, you know that the original is early 2nd century or earlier -- which is fairly important, in and of itself, when it comes to anything relating to Jesus or Christianity. If you have an 8th century papyrus, you know that the original is 8th century or earlier -- which means very little.
So you have to look for other markers of age/importance/authenticity, such as vocabulary words that were in common use in some specific era. Or markers of inauthenticity, such as... well... a lot of stuff in this article.
Can you elaborate on some of these things and provide some reasoning as to why they are markers of inauthenticity?
The short answer is, the manuscript has no traceable history, seems to have come from someone with motive to create a forgery whose story is inconsistent, was presented along with a forged papyrus fragment of the Gospel of John, and is physically questionable -- the handwriting, shape of the piece of paper (probably cut recently), the quality of the ink, and even the grammar are suspect. It has some grammatical weirdness that appears in a particular, modern, translation of the Gospel of Thomas and nowhere else.