Calling it an Idiot Ball is a little harsh. If his prior was that Fremen were pretty worthless, and then a unit of supposed Fremen - who conveniently have Hawat among them - defeat Sardaukar, it's perhaps easier to explain it by supposing Atreides soldiers wearing Fremen clothes (in fact some of them were just that, since the Atreides group under Hawat did join these Fremen) than to suddenly decide the natives were equal to the Imperium's most elite fighters. Especially since the Baron had just done something similar, letting Sardaukar fight in Harkonnen uniforms.