In a statement on their website, Bungie have claimed that the reveal of an MMO project at GDC was in fact a “joke”
“Now, in rehearsal Aldridge was convinced that everybody got the joke. It was all in the delivery, he assured us, and he was certain it was clear that he was playfully riffing off of the recent rumors.”
Some are speculating that this is, in itself, a joke; a cover up.
Thanks to PushSquare for the info.