Extremely skilled players that normal players can't hope to emulate and perhaps who've had a lot of practice on geared toons doing these encounters previously, maybe? Also, didn't they put out a "balance" patch recently, did that not nerf some of inferno?
The key Inferno nerf was that rare mob damage would not scale with # of players - which is irrelevant here since this was done in solo.
The thing is that rare mobs in D3 are pretty much a gear check (though not necessarily so, as this char cleared all but two in the entire Inferno playthrough). Most bosses are a skill check, as this video also showcases. In that sense, D3 is a lot like WoW (though in WoW you also have enrage timers to beat). Standing in fire = retard. Same reasoning.
The Butcher is perhaps the perfect example of this in D3.
Avoid the hook, avoid the fire on the floor, avoid the melee swing. It's an encounter that you learn. Now that I've done it a bazillion times I think it's cake. But I struggled a lot on my first kill.
If you watch the vid, you can see that there is no "extreme skill" involved, just standard play using key abilities. This is exactly how the game was meant to be and I am perfectly happy with it being so.
Take Diablo on the Inferno difficulty. I killed him for the first time when my gear was much poorer than it is now. Yet a while ago, I went back and died to him. I thought: "Fuck this, I cannot be fucked." Why? Because it is a long and intensive encounter in which you cannot really screw up much. Having good gear simply just gives you the additional "Oh shit I fucked up" button.
To be honest, when I was undergeared this was very characteristic of my D3 experience. Do I play perfectly or do I die? In this situation both the skilled and the geared player are rewarded.
Now what would the alternative be? What do you suggest? Have another difficulty (Inferno) with no level crawl included that would simply be a breeze? What is the point of that? The fact is that Inferno does take skill. Yes, you can outgear it, but it does not change the fact that Inferno does take skill.