Kymer's conjecture does
have evidence but it is rational rather than empirical evidence.
We know that intelligent beings exist.
We know that such beings can not only master mechanical engineering but subtle invisible forces like electricity.
We know that much of that reproduces the alleged phenomena of magic.
We know that such beings can master flight in a variety of ways, including interplanetary flight.
We play the music, tell the stories, reproduce the artwork and extol the virtues of primitive societies we mopped up. We are a bit sad we absorbed them so quickly.
We have some conjectures, not proven, but compatible with scientific knowledge that faster-than-light flight might be done.
Simply, we know these things because we have already done them. Except for that last one there.
We have probabilistic conjectures that planetary life may exist elsewhere and last long enough to have a species with the above characteristics.
Given all these knowns we have not inconsiderable Occam weight
But as the Greeks put it, on the other hand
We have seen earthsized planets in stellar systems that did not have a jupiter anchoring them. That is, Jupiter is so huge that it takes up 71% of the mass of the solar planets. Saturn takes up 21%. Earth .3%. All the planets have to be losing orbital velocity from interstellar hydrogen. This happens in proportion of mass to crosssectional area. Jupiter slows the least. Each time Earth passes Jupiter it gets an upward tug that counteracts that. So we have detected at least one solar system where an earthsized planet has drifted downwards till it is hot as our Mercury. Microbial life may have arisen but it is gone now. One of our knowns is that a planet must have a stable environment for 4 or 5 billion years for life like us to arise. There won't be many.
We also treat space flight as an extension of sailing. But what does that cost? People have taken a rowboat across the Atlantic a few times. Sailing is dirt cheap. Unless interstellar flight can be discovered to be that cheap it's not going to happen except as an experiment. It's fun to pretend that an anachronism like stem bolts will be valuable to ship but think of it, with the technology of the Star Trek future they could manufacture anything at all in a local solar system.
I've spent enough time and space on this post. I'm sure you can all come up with other realistic improbabilities that way against ancient contacts.