In other words, it's intended as a continuation of the Bronze-Age Superman, but told in a new style.
Grant Morrison says that, but I really don't see it. It reminds me of any given magazine interview with Spike Lee: he mentions all this subtext in his flicks, and my reaction is, "whoa, whoa, whoa, where was THIS in the movie?"
For every interview with Grant where he says something like that, there are other interviews with other people where they give the "goal" and "chief source of appeal" of the ALL-STAR line being that they restore characters to a classic status quo.
The thing is, Grant worked very hard in ALL-STAR to give the All-Star Superman a definite, distinct identity that belongs to that book alone.
(For the most part, I find what he's come up with fascinating.)
This does not necessarily mean that ALL-STAR Superman is his own thing apart from the Pre-Crisis Mythos...but that claims that ASS Superman is "a continuation of classic Superman" have to be examined more critically. There's a literal continuation...and then there's a continuation in spirit as well: some may say Busiek's ARROWSMITH, about a boy that grows to be a man/hero with help from a heroic older mentor, is a continuation in spirit of TERRY AND THE PIRATES, but ARROWSMITH is not literally a "continuation" of TERRY AND THE PIRATES (the most obvious reason being that Arrowsmith is set some time before T and the P happened).
Now, compare ALL-STAR to, for instance, Johns and Busiek's SUPERMAN and ACTION COMICS, which explicitly built on what was going on with One Year Later and INFINITE CRISIS (with Supergirl as defender of Metropolis and Superman depowered, Lex a crook as a result of his plan in 52 falling through, etc).
The events that Grant Morrison chooses to build up and elaborate on, are events that Grant HIMSELF placed in the background of All-Star Superman: clearly Dino-Czar and Superman have met before; Samson wasn't a time traveller last time we saw him, and the significance of Leo Quintum and the Project is based on developments we the reader didn't see.
You are right in the case of the Fortress Key, which was interesting, however, there are other details that suggest also that this is NOT a literal continuation from Pre-C Supes: for instance, Steve Lombard shows up in the first issue with all his "classic" macho swagger, when in fact in the late eighties he had been humbled and become much less of a jerk.
Most likely it isn't the case that Grant Morrison is EITHER scrapping the Pre-Crisis universe and building his own thng, OR that he is continuing it directly, but rather, he is using what he wants (the Fortress Key) and ignoring what he doesn't want (Steve Lombard mellowing out, Lois Lane no longer being interested in Superman).
You are of course, correct that this approach isn't entirely the same as making a clean break, but it is also true that "cherry picking" continuity like this isn't the same as a continuation, either.
And it is also true that this approach is used to give characters their classic status quo. There's nothing necessarily wrong with this, but I do have a problem with this attitude because characters are the result of a process. In ALL-STAR SUPERMAN, Lois wants to be Superman's girlfriend, and Superman feels the same...which is their "classic" arrangement. Jimmy Olsen is a teenager instead of being the young adult "Mr. Action" he later was. Steve Lombard is back to being a jerky guy.
That's where my problem comes in: the idea that characters can progress (e.g. Dick Grayson being an adult and the leader of a team) is somehow
inconvenient.