You are right. I should have qualified my statement by sayingI'm paraphrasing, but it's an interesting point he makes that leads me to believe that Acererak is not the BBEG as might be expected.
So I suppose it's possible, but I don't think anything is a given.
It's all but a given he will be [watered down] if he's meant to face the adventurers and lose to them.
Of course, having the adventure point towards Tomb of Horrors (and his undiluted stats) is entirely possible.