Although I wasn't there when the few stealthies actually performed this stunt I saw the effects. The teleport pad before Mimron's room only activates after Mimron has been engaged for the first time, fortunately, the trash is able to be stealthed past by some combination of druids/rogues/mage(invis). Then the stealthers simply engage Mimron and either wipe or reset through vanish. Once that is done the rest of the raid can poof past all the trash!
Note: if your guild is going for the "Champion of Ulduar" or heroic version where no raid member dies during an encounter, you'll want to make sure that the stealthers don't wipe but vanish to reset it instead.