Jude did not quote the book of Enoch1, but rather attributes the quote to Enoch, a specific individual we know that lived before Noah2.
What Jude does do is he amplifies Enoch, the man. We know he was righteous all his days2,3, but now we know he was also a prophet of God (or spokesman for God, or speaking God’s literal words). As so many other New Testament/inspired writers do, Jude applies and defines a prophet’s prophesy in the context of the New Testament gospel message. A divine and inspired interpretation of a prophetic saying. In other words, no guesswork required by us!
The fact that this prophesy isn’t recorded elsewhere in Scripture isn’t relevant (or without precedent – John 7:38). But rather, just as a true prophet would speak, these words are completely consistent with other statements about God’s judgement on false teachers.