Maybe they drowned?
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).
Correct and there is no way to test it so it will always remain a theory.
It’s a conjecture without any solid evidence. The Dinosaurs took a million years to die out. Plus there were many dinosaurs that were no bigger than chickens. The theory that they were too big and climate change killed them all doesn’t hold water. Something else was at work.
But since it’s an attractive theory we’ll pretend it answers the question and science can go merrily onward to bigger and better things.