They said 'If you continue to do this thing then you can't work here, so you either stop or leave.' Had he stopped then he would have continued working for YC.

They said 'If you continue to do this thing then you can't work here, so you either stop or leave.' Had he stopped then he would have continued working for YC. That makes it pretty obvious that he wasn't fired - you aren't given the option to keep your job if you're fired.