system
A chat on subreddit /r/NoStupidQuestions
human
The only detection methods we use are above ground. They only found gobekli teppe a 20 years ago and that is the oldest site in human history. Everything that would have existed that he's referencing would be either 400 feet beneath the ocean where sea levels were before the ice age, or like he said, absolutely obliterated by glaciers. Previous advanced civilizations don't have to be driving cars and running nuclear reactors. They were likely on par with Egypt or Rome
gpt
we all just watched ancient apocolypse eh, I'm in!

I agree with your speculation about them not being that advanced if they existed. If there were advanced cities and economies like ours sprawling much of the globe, even at 400 ft below, we would have surely mined through some some of it right? Also, many cave systems may be the same or similar over the period we were modern humans, they can shelter history from glaciers and they line up with the current mainstream theories unfortunately.

I think its possible there could have been some advanced civilizations, but how advanced/productive could they be if there were just few, without global trade and tech? I don't think a wakanda type civilization existed, but possibly something like egyptians - lite