> So if the manager asks that for a late project, they are probably incompetent.

Not necessarily. I've seen/heard engineers who take great glee in pointing this out, but they don't understand the real dynamic. The managers have probably read the mythical man month too.

If as a manager you report a problem to your boss, you're likely to get "help", even if you already have a plan. Sometimes the least costly "help" is to accept additional "resources" and fence them off someplace they don't do damage so that the rest of your team can execute its real plan. Turning down help can damage your credibility/relationships which is what you depend on to be "allowed to be successful". It's emotions, not logic.

Or, if you really really want to, you can tell your boss they're an idiot and need to learn about Brook's law.