Your assumption that a snippet of Jude quoting the Book of Enoch legitimizes the entire book would have some validity if we can now canonize every remark by the Cretans, since Paul clearly quotes them telling him, “Cretans are all liars” Titus 1:12.
Paul quoted several Greek poets. “A some of your own poets have said, ‘For we are all his children’”.
Later Christian first century authors also quoted Homer’s writings.
Jude simply quoted from a text of Enoch.