although beating Sidious in a lightsaber duel is gosh darn impressive
Windu did not actually beat Sidious. It was just a part of the plan to turn Anakin to the Dark Side. Before Anakin entered he let himself be disarmed so he would appear like a victim.
I mean I know this was kinda "left to interpretation" so the kids can be content but anyone with a little bit of intellect or common sense can see that it was obviously staged.