if all dolphins were wiped out could we know that they were ever here from physical evidence?

We’ve already got fossils of dolphins. Not exactly the species we know today but close enough. We have enough fossils to trace their evolution since their ancestors walked on land. So yes, we definitely would have evidence that dolphins were here.

We’ll never know of the existence of most species that ever existed. Fossilization is rare.

right, so how can one easily say this hypothesis is false? "advanced civilization" seems to be strangely characterized. would something a few steps above beavers be advanced enough?

How can anyone say the hypothesis is true? There is absolutely no evidence for it. It’s just a thought experiment.

When people speculate about "advanced" ancient civilizations they usually mean said civilizations had technology at least as advanced as our own if not more so like nuclear weapons, antigravity, modern manufacturing and engineering, etc. A hypothesis that millions of years ago humans or some progenitor could build rough shelters out of sticks isn't particularly interesting.