> And someone is saying there could have been an entire advanced civilization 400 million years ago, large and powerful enough to cause climate change, and that no traces of it would possibly be left?

If humanity vanished today, in 1 million years almost all of our artifacts would be gone or burned under miles of subduction.

In 10 million years there would literally be nothing left.

> If humanity vanished today, in 1 million years almost all of our artifacts would be gone or burned under miles of subduction.

That is completely incorrect. We have sedimentary rocks from 100 M years ago on the Earth’s surface right now, that never underwent any kind of subduction.

Bollocks. We find fossils millions of years old right on the surface.

> under miles of subduction

Subduction only occurs at plate boundaries and is not nearly this fast. Continental plates float on top like foam and do not get subducted. It's the oceanic plate that is created at mid-ocean ridges and gets subducted at the margins.