It's 100% not required and 100% a gimmick. There's no way whatsoever that Blizzard will tell you "get a priest or go home". Another mythic-only mechanic is Slag Pool, and I'm almost positive you'll need to tank them inside it. But I guarantee you that there is no chance at all that you're expected to mind control the adds (at least not with a priest's Dominate Mind, they might choose to add some sort of Mind Control Orb a la BWL) -- whether you will be able to is a different argument.