There were previous extinction events at the end of previous glacial stages. But none as massive. Also this most recent one affected primarily large animals, and all of the largest died out. Previous such events tended to affect animal species of all types, and it’s difficult to imagine how a “natural” event would target large species.
I put scare quotes around natural because if you believe in evolution then human-caused extinctions are as natural as those caused by any other factor.
If a large number of large species had been around for two million years, including more than a dozen glacial cycles, then all disappeared within a few centuries, leaving most smaller species unaffected, within a short period after human arrival, that is a truly amazing coincidence.
I suspect the weak link in the “humans did it” theory is that humans may very well have been around for a lot longer than the theory allows.
The massive slaughter idea would be more logical if the ancient Indians had AK-47s. But they apparently didn’t even have bows.
The animals being “naive” about the danger of humans might allow you to get really close, close enough to use a spear, but it’s not going to be much help once you put your spear into a mammoth or a short-faced bear that doesn’t die immediately.
I still find it hard to believe humans had any more effect than any of the other predators. They obviously were not the top of the food chain. They also probably only killed one animal at a time, despite evidence of buffalo jumps. I once got a deer so far back in the woods I had to get a wheelbarrow to bring it out halfway, lol.