Aaaand I'm an idiot. That always baffled me though. I understand rounding down for stats and the like, but rounding down for costs made no sense to me.
Anyway, you're right, but I'm not sure it's worth the extra base power anyway. His examples make no sense. No coverage if you run Flail, Sub and QA and SD seems to me a bit pointless if the coverage would let you hit those things in the first place: Magnezone fears Earthquake more unless it uses Magnet Rise. Skarmory is better dealt with by switching to a special attacker, and Scizor can be dealt with by switching, preferably to your own Magnezone (In fact, why has he included it in his calcs? That applies for any set with QA and SD). If there are real threats which Flail can take care of that Return can't (i.e OHKO rather than 2HKO) than it's worth consideration. Until then...
Well, 1HP Flail has a Base Power of 200, meaning it'll do about twice as much damage as Return, meaning it'll OHKO practically anything Return 2HKOs.