The RPGA Tomb of Horrors update/remix can be thought of as taking place "before" the Tomb of Horrors super-adventure. However, it was wholly inspired by the super-adventure; not the other way round. It's not a necessary preamble in any way to the super-adventure, but there is some continuity between the two for DMs who want to make use of it.
The backstory of Bruce Cordell's extraordinary 2e box-set adventure Return to the Tomb of Horrors is part of 4e canon courtesy of Open Grave, which talks about Acererak in the context of the plots and mad schemes that drove RToH. That canon was further expanded for 4e in Rob Schwalb's "Legacy of Acererak" in Dragon 371, which talks about Skull City and the Tomb in a kind of current-to-your-campaign context. All that backstory is the starting point for the super-adventure, which blows that backstory wide open. Neither Ari Marmell nor i are that interested in giving up any details of the super-adventure (though you can probably expect some previews on DDI). However, suffice it to say that Acererak's plots are going way beyond anything that's come before.
Knowing that the super-adventure was going to mark a turning point in the legend of Acererak, i thought it would be cool to revisit the original Tomb for 4e, setting it up as a kind of adjunct to Rob's excellent Dragon piece, and with some consistency to the amazing story that Ari crafted for the super-adventure. What this means in campaign terms is that the RPGA Tomb update represents the Tomb as it is "now" in your campaign world — at the point just before the events of the Tomb super-adventure shake the legend of Acererak to its foundations.
One other big, big difference between the two projects. The RPGA update was done mostly just for the sake of doing it, and to show how one old-schooler (me) would take on the task of updating one of the most legendary AD&D dungeon crawls for 4e. I think people will enjoy it (both those who know the original adventure and those for whom this might be their first real experience of the Tomb), but in the end, it's kind of a one-off bit of fun.
The Tomb of Horrors super-adventure, on the other hand, will seriously blow your mind.