Since the world is full of pretty poor managers "don't swear at your boss" is pragmatic advice. It'd be nice if all managers had the skill to recognise that a bit of venting is probably healthy.

"Don't work for managers you can't swear at," would be my advice to the world :D

That's good advice for people who are in a position to chose who they work for. Is OP in that position?

Yeah, but there's a lot of insecure managers out there, and if you're desperate for a job, you need to take that into account.