If you define aliens as a long separated branch on the tree of life with significantly more advanced technology, then alien encounters are more common in history (hawai’i in 1790’s being most recent i can think of).

So mathematically its rare, but perhaps an experience of 1% of societies . And perhaps strongly correlated with one branch undergoing a rapid technological advance.

i think it’s perfectly rationale and not fantastical, as long as its kept in mind that its a low probability event.