It's very far from obscure. But the editions themselves shift.
In the first edition version, the only offensive abilities Acererak had manifested if the players attacked the phantasm or (Idiotically) touched the skull. The text makes clear that these are the only conditions under which...
In continuity terms, I assume that what we'll see presented as the default scenario is that Acererak has survived the events of Tomb of Horrors, Return to The Tomb of Horrors (2e) and those of 4th ed Tomb of Horrors, getting his schemes foiled each time, and is now back at it once again...