> "If it's a copy, it's not relevant for dating the book."

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.