Let's get all the information out there so people can judge for themselves. These are simply the facts of what happened. I am leaving my opinion on the matter out of this post completely.
In phase 2, Diablo summons shadow copies of each group member. These shadow copies used abilities that could one shot Method's tank(s). They were incredibly powerful and nearly impossible for Method to survive more than a few seconds after entering phase 2. After a few wipes, they began trying something new. Method would have 2 party members purposefully die at the end of phase 1, then resurrect those members, but have them wait until after phase 2 had started to accept those resurrections. This caused the game to only summon 2 shadow copies, and they could control which classes were summoned. The shadow copies despawn after about a minute, so after entering phase 2, they would kite the copies for a bit, have one of the dead people accept the resurrection that had already been given in phase 1, continue to kite the copies, and then, after the minute was up and they had despawned, have the fourth person accept their rez. They repeated this process twice (total of 3 times) during phase 2 when he summoned shadow copies again. Method was completely unable to deal with four shadow copies, so they started using this technique in order to get through phase 2. In addition to this, because of the purposeful dying, they were dying enough that their gear was breaking. They received gear from team GD to use as a replacement for the gear that broke during the encounter so they could continue even after dying more than 10 times. All of Method's members received extra Inferno level gear from GD, but they did not all use all of the gear during the encounter and so may have been able to complete the kill without it.
Method has the first completely confirmed kill, as they streamed it live. The previous kill of Diablo Inferno solo by a wizard has only been screenshots. That person also used an interesting technique to defeat Diablo. He used Energy armor with the Force armor rune, which makes it so any damage that does more than 35% of his health in one hit is instead reduced to 35% of his health (i.e. boss hits you for 100k, you have 100 health, boss only damages you for 35). This skill/rune combo, combined with getting as little gear as possible with vitality on it (so you have very little max health), getting lots of health regen, using Magic Weapon skill with Blood Magic rune(2% of damage turned into life), and using the Galvanizing Ward (Energy armor lasts 4 minutes and regens 310 hp/sec while active) passive allowed him to take absurd damage without actually dying. Again, we don't have video proof of this kill, but this is the tactic that would most likely have been used based on the gear and skills shown in some of the screenshots.
So: one confirmed kill; one unconfirmed, but likely true kill. Both kills used techniques that were likely outside of what Blizzard had intended for the encounters. I'll leave it up to you to decide for yourself whether you think either, both, or neither of these kills was legitimate.