Ah yes, gatekeeping and name calling. The cornerstone of "established" science everywhere. There is no need to denigrate Graham Hancock's hypothesis as "crackpot" and it certainly isn't a proper theory. Science welcomes all ideas only to discard them with proof to the contrary. Unfortunately, much of archeology is inference not proven fact. The farther back we go the more inference and less fact we have. There is certainly a possibility that an advanced Pleistocene civilization existed and was destroyed by a cosmic impact. To dismiss these ideas as "crackpot" is one of the most unscientific claims I regularly encounter.

We know there are human built megalithic structures that are dated at the end of the Pleistocene. We know that the Pleistocene ended abruptly with all North American megafauna perishing and immense amounts of meltwater entering the oceans. We don't know if there were a bunch of distributed civilizations with similar technology levels or if there was a global civilization that shared knowledge before or after cataclysm. We also don't know what caused the end of the Pleistocene. This is the realm of hypothesis on all sides.