You are forgetting the ruins...every trace EXCEPT THEM were vanished.
Except you said earlier:
Had you watched the video, you'd have seen the evidence of a massive debris wall snapping off mastodon femurs.
So, if there is evidence of a "massive debris wall" that can show us snapped off mastodon femurs, then why is it not showing us actual bits of the civilisation in question other than these buildings?
Or are the other buildings, roads etc somehow more fragile than a mastodon leg? Or more prone to some sort of sorting?
Under water, perhaps? That tends to happen with flooding.
Considering we have ruins, we would expect the other stuff to be located near them. That's how pretty much all other sites work.