Yeah, any number of civilizations could have existed before that that wouldn't be in the ice record.
(I personally don't think this was the case and think civilization of any kind is a recent phenomenon in the last 50,000 years or so, but the Silurian Hypothesis exists)
What if the pyramids, a potential Richat city, etc are way older than we think and are actually from 30,000+ years ago? How possible is that? I.e. how possible is it that 30,000 year old pyramids would still be standing?
I'd err towards all of the above being much more recent, especially if people in 500 BC still had historical legend about Atlantis being not too distant from them.
The guesses I've seen at Atlantis' timeframe have been: 30,000 years ago, 18,000-12,000 years ago, 5000 years ago. They're all over the place. Was the 12,000 years ago flood THE great flood of Noah, or was Noah's a second, later flood? The Bible's got it happening at what, 3000 BC? Yet we've got the north atlantic glacial maximum melting down, and a likely massive explosion of water, at 10000 BC or so.
Kind of cool that Atlantis has become such a huge thing for me to mull over lately, after decades of always hearing about "the lost city of atlantis" and thinking it was just some fable people made up in the 1500s about a city that fell off a cliff or something, who cares. Turns out it might have been a way bigger deal, if it was the forerunner for the Egyptians/Greeks/Romans and the pioneer of travel/mapping/trade routes in the Sahara / Atlantic region.