As one of your sources said, it's not all that remarkable. If you were going to write something purportedly by a pre-Flood author, you would of course omit things that happened later. It's easy to leave things out. More significant are those things that are included in Enoch. And looking at them, there is nothing remarkably prophetic. Unless, as you say, a fragment can be decisively dated to a time that would make such writings miraculous indeed. But so far, I see no need to leap to unwarranted conclusions. I'm always open-minded to solid evidence, however. Keep me advised if something turns up.
Using your standard of measure, thus far the eyebrow-raiser would be Enoch fragment 4Q204 which appears to be carbon dated about 180 B.C.
I'm searching for more information, but the anamoly is that orbits were not proposed by the Greeks until 150 B.C. coupled with the fact that the Enoch manuscript is a copy from a source of unknown antiquity.
Finally, there are fragments just now being made available for carbon dating - so more questions may arise.
(That's a paraphrase of what I've found thus far; I will try to find quality links and excerpts to back it up.)