I've never been a big fan of treating time as if it has some kind of "sentience" or ability to "correct" itself, in the case of paradox, etc. or somehow "alter" the way things are when you left, etc.

If UFOs were really caused by time travelers, then it would mean of course that yes, time travel can interfere with the past. What are the ramifications? Maybe nothing, maybe everything? Who knows.

Still though, I think the evidence leans more towards the idea of an alien origin than a future one. Lots of stars, lots of planets, I seriously doubt we're the only ones, and we're also likely not the oldest or most advanced ones out there...