Of course a 10x dev understands the existing system, but it's more than that. A 10x dev IMO, understands how to logically take apart and put together a system. I find it amusing that we recognize grand masters in chess and elite performs in other fields, but don't wish to acknowledge that such people exist in our fields.
What I have seen with such developers is that they can jump into a system they have no idea about and ramp up faster than most people. I don't believe that everyone can be a 10x, I also don't believe that everyone that can be a 10x is even going to reach that field. From what I have seen, these folks are the sorts that enjoy working on non user centric app. They hate those ambiguities, so you might find them in security, or working on compilers, networking and lower level sort of codes where it's possible to conquer whatever demon that will arise from the code. It's almost impossible to WIN in user centric app where what's correct is subjective.
What I have seen with such developers is that they can jump into a system they have no idea about and ramp up faster than most people. I don't believe that everyone can be a 10x, I also don't believe that everyone that can be a 10x is even going to reach that field. From what I have seen, these folks are the sorts that enjoy working on non user centric app. They hate those ambiguities, so you might find them in security, or working on compilers, networking and lower level sort of codes where it's possible to conquer whatever demon that will arise from the code. It's almost impossible to WIN in user centric app where what's correct is subjective.