It's an exploit. It will be fixed (if Blizzard doesn't fix this, people will be clearing GR150 by the end of this week, which "breaks the game"). Expect those people to be banned, or at least major rollbacks to rid everyone of the excessive paragon/gem levels gained.
You're not supposed to one-shot rift guardians like that, this has nothing to do anymore with clever game mechanics. Remember the Hellfire amulet bug, the blood shard exploit, Death's Bargain exploit? It was fixed and accounts were sanctioned.