That would imply doctors couldn't set a broken bone in the Revolutionary War? The Civil War? Of course they could! They could even do amputations.

A broken leg could be fatal if the broken bone shard cut the artery. That can happen even today.

But plenty of people in premodern times survived leg fractures, though often at a cost of a lifelong limp.

It's not about the initial injury. How are you going to find food when you can't move properly? How are you going to avoid predators? If you are alone and not in a group that can help you with these things until you heal it's pretty much a death sentence.