That's wrong. He had his elemental powers which is what we were discussing to begin with.
After Garrosh taunting him for having twisted the elements Thrall manages to call to them by casting a Thunderstorm that dispells the void corruption while yelling " Never powerless Garrosh. And never alone".
He afterwards is still no match for a old god power infused Garrosh, but regardless that would happen just the same with Varian.