It is still theory. What they came to realize about a decade ago is that a bunch of ‘critters’ (mammoths are a great example)...died in a sudden manner. We aren’t talking about a hundred years, or a year, or even a month...but an event that pretty much wiped them out in North America in a matter hours/days.
A lot of this speculation goes back to this meteor event and the rapid evaporation of the North American glacier....during the Younger Dryas period (12,900 years ago).
Fractions of a second, in geologic time.