Harry leaps to that conclusion before hearing from Dumbledore how difficult they are to create. Even if that was the method, there is still the question of how they managed to accomplish it.

My hypothesis — as of several chapters ago — is that Dumbledore assisted in the Rita prank. He certainly had the motive, since he's playing the game against Lucius and Rita was Lucius's pawn. He also had the means (being incredibly powerful). Why hadn't he acted against her earlier? Because he hadn't been clever enough to think up a good way to get at her without inviting retaliation.

So how did he ever get included in the twins' plan?

Easy: he's in the habit of routinely reading their mind. Evidence for this lies in chapter 63: "It wasn't that the Headmaster had popped up out of nowhere and was staring at them with a stern expression. Dumbledore was always doing that." There's also weak evidence in chapter 12, where Dumbledore knows Harry wants to reformulate Quidditch (he could know via F&G via Ron). And in chapter 79, where he knows about the map.

So: The twins are walking around thinking about how to implement their plan against Rita, Dumbledore pops up out of nowhere looking for some good gossip, sees their plans, seizes the opportunity. The exact implementation could either be a memory charm (maybe trap her when she shows up at Mary's room looking for gossip about Amelia Bones, Dumbledore's ally), or else Dumbledore could actually pull off the acts Quirrell calls impossible.

The twins didn't brainwash Rita, they paid somebody to do it for them.

was it worth it to make such a hubbub with such a supremely, well, boring answer?

Yes. That was the point of the whole incident.