I think the general point is that a big population crash may well leave a remnant which is unable to re-industrialise (too few to make key inventions, too scattered, too many other problems). Or the remnant may have strong cultural pressures against re-industrialising, given it was such a disaster last time.
The small residual population would eventually go extinct from natural causes, or evolve into something non-human. So the crash is an existential risk.
I think the general point is that a big population crash may well leave a remnant which is unable to re-industrialise (too few to make key inventions, too scattered, too many other problems). Or the remnant may have strong cultural pressures against re-industrialising, given it was such a disaster last time. The small residual population would eventually go extinct from natural causes, or evolve into something non-human. So the crash is an existential risk.