It's not (just) smug mockery. It's taking a post that might have made a big impression on people, and giving a deliberately silly variant to make it clear how the logic is incorrect.

Just pointing out the faulty logic would've made for a boring post that many people wouldn't have bothered reading, and would risk being dismissed as hand-waving or rationalizing. Picking something eye-catching and obviously ridiculous keeps people's attention, and also can't be dismissed as mere rhetoric since the conclusion does actually hold.