They can’t use the old batching. That’s not possible. The old batching was an artifact of the old server mechanics, and since they’re not using the old servers, using the old batching is impossible.
Modern WoW doesn’t have “spell batching” in the sense that we’re talking about here. It’s technically the same system that Classic is using but it’s set up differently to the point that it’s basically a different system. What we’re discussing is the attempt of Blizzard to recreate the Vanilla mechanics using the modern server mechanics.
You can read more about the spell batching being used in Classic here: Spell Batching in Classic