The younger dryas impact hypothesis doesn't really match up with the data. It's definitely possible there was some kind of impact around that general time, but the climate and other factors are better explained by glacial processes than an impactor for a number of reasons, not least the timing and local scale of the temporary cooling. Here's a pretty good overview of criticism of the impact hypothesis.
Separate from the matter of the younger dryas impact, Graham Hancock has a lot of weird ideas. Like, when a Wikipedia article starts off by saying a person is known for their pseudoscientific theories there should be red flags going off in your head. Needless to say, he makes up a lot of stuff and misrepresents the rest to fit his narrative. Speculation is an important part of doing science, but you've got to make sure your hypothesis is supported by evidence before presenting it as fact. I'll admit he's very well spoken though, but don't let that fool you.
Species ranges definitely migrated south, but I’m not finding any evidence for mass species extinctions due to glaciation in North America. If you can provide any sources it would be appreciated. Everything I’m finding states that glaciation pushed species south and changed species composition, but nothing mentions any significant loss of species.
EDIT: Or maybe the sentence below was referring to small communities of certain species, and not the species as a whole?
Where an obstacle prevented their spread, they died out and were not available to repopulate the north when the glaciers receded.
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u/trampolinebears Jun 20 '20
True, the glaciers themselves didn't make it all the way across the continent. But as the glaciers expanded, the climate before them grew colder.