I don't think you can dismiss the possibility of an as-yet-undiscovered end-run around these limits. The problem with a critique of a hypothetically more technologically advanced civilization than our own which is based on modern understandings of physics is problematic at best.

There are larger problems with the ET hypothesis, though, and that is why generally, descriptions of beings associated with lights in the sky, although also often associated with knowledge of new technology and with kidnapping, are described in physical details differently by different cultures. Even in somewhere as narrow 10th century Europe, you have at least three different, if you will, species of entity associated with this sort of thing.

So this leaves the ET hypothesis with two bad choices, which are either we are more observant than our ancestors which is patently false, or else there is some intersteller convention somewhere which divvies up cultures for observation and follows them as they move around, and stops when the culture changes sufficiently. That starts to sound very implausible.