At least they did manage to get global warming in there.
The atlatl is quite efficient.
But I dont think that was the decisive tool.
I think the root problem is that people were way smarter predators than anything these beasts had to deal with before, and their reproduction rate just wasn’t up to balancing the increased mortality.
Same deal with the extinction of large beasts in North America. Better predators showed up and hunted them out.
The same thing happened, already in historical times, to the giant moa birds of New Zealand.