I agree that he shouldn’t have said that. It must be done because, when the US goes broke we are going to end up firing every one of them and not just trim the force.
The fact that he said it means that he knows he can’t win, so he’s just saying something controversial and putting it out there. Trump may well trim the federal payroll considerably. But it’ll take generations to slowly attrition the massive bureaucracy into something livable. It didn’t suddenly appear one day. It’s been growing steadily since 1933.