Very skeptical that P6’s biggest platform would be PlayStation if the game received a simultaneous multi-platform launch. And the rumors of its exclusivity seem to have their origins in very inconclusive posts on Era without much evidence.
Could still happen, but not something I’d bet on right now.
I am doubtful, but I also think there is little chance of this ever being a scenario tested in reality.
I can't see P6 not being a priority for SIE and I don't see ATLUS having much desire to say no to a deal.