The one thing I think you are not considering in this is that Jude is not writing these things based on his own understanding of history or oral or written records. What he has written he was directed to write by the Holy Spirit. It makes no difference whether the prophesy of Enoch was even known by man before this or not. The Holy Spirit was the one who gave Enoch the prophesy in the first place. Now, He is relaying this same prophesy to Jude and telling us that this was also prophesied by Enoch. Enoch never wrote anything since he lived at least 1000 years before the developments of any written language.