Megafauna is relative.

If you're talking about large terrestrial mammals that coexisted with prehistoric humans and died out at the end of the pleistocene, probably not.

I think it's unlikely that humans existed in large enough numbers to exterminate entire species, especially given the time it would have taken humans to really proliferate in the Americas.

Humans were probably an aggravating factor, but I don't think you can conclusively say that animals like the the wooly mammoth or rhino would have survived the climate change that began approx. 12 000 years ago.

It's all too coincidental with climate change to just blame humans, especially considering a whole bunch of species similar to those that died out were able to survive alongside humans.