HAHAHAHA! Yeah, nope.
Humans are ridiculously good at killing things.
So you are 100% correct that an ordinary stone age hunter against a bear, with nothing but his bare hands, is going to die.
But a stone age hunter is going to run away, if at all possible, go back and get his spear and a bunch of his buddies and their spears and then go kill the shit out of that bear, eat it for dinner and wear it’s skin to let the rest of the bears know not to mess with him.
Even if he can’t get away and gets killed by the bear. His buddies will probably find his remains and realise there is a dangerous, and tasty, bear in the area. They will then hunt down and, again, kill the shit out of that bear.
The biggest issue is that we can remember, plan, and coordinate. So let’s say you are Mr. Bear, and we kill one of your cubs. If we get away, you are liable to be angry for a while, but you will eventually forget about it and, the next day, you will go out foraging for berries and nuts and whatnot. But, if you kill one of our kids, we will not only remember it, we will tell other humans about it, and we will then collectively hunt your ass down and kill you and any other big furry asshole in the immediate vicinity, just in case one of the innocent bears gets the idea that killing humans is cool.
It’s Human intelligence and human tools that make us dangerous. Not human fangs and claws.