I think you've glossed over the fact that when you get to this level, with a few exceptions only, you can play any class fantastically and not just your own - therefore, alts are not a hindrance but a benefit if you have enough people at that level where you can raid the content on them to keep them at identical gear level. It's what Blood Legion did in Firelands, didn't they have like 6 Firelands 25man hc runs going to pump out as many dragonwraths as possible? Something ridiculous like that.
For a more normal comparison, remember that one really annoying guy in your guild who had alts available but would still top recount or always survive on them, despite being a bit undergeared and not spending as much time on it? The top guilds are comprised solely of those people. I myself have got rankings on alts back in WOTLK/Cata and was up to playing a couple to 25man hc level, but I still wouldn't put myself in the same class as those guys. They're a different caliber.