While this may be an encouraging sign, there are questions that I didn't see answers to. Such as What was the after effect to the grouper from eating the venom of the lionfish?
Second point: It may be that the groupers are now eating the Lions because there is no other food fish available anymore so "desperate times call for desperate measures" but if the grouper had any ill effects digesting the fish, I doubt it would eat another one.
Third point: These groupers are native to the areas where Lionfish first were introduced. If they were the answer, why did the lions get so out of control in the first place?
Just some thoughts...