Acererak is sort of like a comic book villain: each time you think he's defeated, he pops up again. Bit like Vecna, really. Or Magneto.
What is particularly interesting about this new adventure is that in the first two adventures, the presumption is that it's in the World of Greyhawk. With the new Tomb of Horrors, it's in the new cosmology and setting - which is very much "the Best of" setting, where if something was really good in a previous edition of D&D, there's a place for it here.
So events such as the original ToH and RttToH may have occurred here, but not exactly as you might imagine them.
(As far as really dangerous adventures go, I still really enjoy Gary Gygax's Necropolis, which I ran during 3e).
Cheers!