Oh no, a
forum mod is denying the existance of a game that they are probably contractually obligated not to reveal the existance of.
M$ really needs Bioshock to help pad up their holiday release lineup, so I doubt they'd be exactly thrilled if 2K went out and told people "Save your money, it's coming to PS3 next year." That's why they almost definitely would have written something about it into the paperwork.
In all likeliness, the forum mod knows nothing more than what his bosses tell him, and for security reasons, he's probably been told the PS3 version of Bioshock doesn't exist.