He had the sporeggar shield and thunderfury (a blue rep reward and an orange sword from vanilla). Thunderfury affected up to 5 targets, the shield affected them all, however probably didn't do much. In essence, he wasn't really "AoE Grinding" so much as he was holding a lot of mobs on him while burning them down in groups using cleave and TClap (maybe WW in there too, wasn't watching too closely). The extra mobs DID help rage gen, but he really wasn't doing much to all of the mobs at once. Honestly, 4-5 targets would have been much better for him to pull, since he can still get the benefits of thunderfury and the full effect of WW/TC, and he could just quickly bandage then move on to the next (if he can handle 40 at a time, cool, but it's not that much more effective to go for extra mobs).
On the other hand, a paladin, who is continually dealing several types of reflective damage, plus consecrate (and considering that holy shield will affect all the mobs at different times, rather than the same mobs over and over like tclap/cleave/WW) would benefit a lot more from having more mobs on him. A mage, another common AoE grinder, would also benefit from having more, as can be seen in the faxmonkey 93-mob-pull. But overall a warrior AoE grinding isn't that great of an idea IMO. This guy is most likely very well geared, I doubt most warriors would be able to repeatedly handle 10+ mobs at a time in solo grinding, and if they could 4 would be the ideal number IMO, since you can't really do much to more than that.