We didn't. The Peter Principle refers to a completely different phenomenon-- "Rise to the level of your own incompetence."

What do people who rise above their own incompetence feel if not imposter syndrome?

Impostor syndrome means the person feels incompetent despite not being. If one is actually incompetent, then feeling like so is just having an accurate perception :)

Plus, I'd say many don't even realize they're incompetent.