I think there's body to this rumor, partly because its popped up in many independent channels of information and partly because it seems like a good move considering how everyone at E3 went absolutely bonkers at that tech demo.
Such a game could warrant more sales than the next installment in the main series, it seems.
I guess we'll see in time, but not this year, and not the next.
I'm thinking more like E3 or GDC 2010 for this, when all three FFXIII titles have had time to simmer and sell for a while.