Sony would not have direct influence over the decisions the board of directors makes. Where it's influence would come from is the board of director's wish to keep the a significant shareholder like Sony happy, without annoying other investors.
Here is what I suspect has happened.
Sony: "Help us out here"
SE: "We'll keep FFXIII and Versus exclusive but you have to shut up and let us do what we want with everything else."
Sony: "We'll do whatever you want, just put out FFXIII ASAP"
starcraft - Playing Games = FUN, Talking about Games = SERIOUS