I like how you say this from the perspective not of a team lead but as another person on the team. Which means you actually don't have any real sense of how much documentation your boss had to do, just what your boss told you they did.
"Sooooo... What's going on with Charlie? Are you documenting? I don't feel like you're documenting." sounds like you hounded your boss into firing your teammate instead of continuing to coach them.
The part in quotes is the lead's manager talking. By the time you're being performance managed, your boss, their boss, their boss, etc. know about it, and the higher you go in the chain the more you're just a line in a spreadsheet that needs to be eliminated. You're on a 100 person team that needs to figure out how to get to 97, and you're the low hanging fruit because you're already marked. What you (as an IC) assume is your boss being a dick is actually a vast mechanism behind your boss, including upper management and HR, pushing them to resolve the situation.
> By the time you're being performance managed, your boss, their boss, their boss, etc. know about it, and the higher you go in the chain the more you're just a line in a spreadsheet
This is so true.
I've never been "performance managed", but if I were to be, I'd immediately start looking for another job because of what you're describing. Even if I wasn't eventually fired, I know that black mark will color everyone's perception of me from then on.
"Sooooo... What's going on with Charlie? Are you documenting? I don't feel like you're documenting." sounds like you hounded your boss into firing your teammate instead of continuing to coach them.