There's a clear distinction between not announcing a game until it's ready and deliberately withholding announcements for the sake of an event. Historically, the latter is not really the approach they've taken with most PSN games which get less fanfare in the first place. And third parties participating in such a coordinated blackout would typically only be the closest business partners with Sony, not the entire lot of them.
Further, withholding the majority of game announcements until an event in May would basically mean none of these games are coming out before that and likely if they're not coming out before May, they're then more likely to be bunched up towards the holiday season. So it would create a huge vaccuum of very little released before that.
Finally, another point Eden made in his presentation is that they're always looking for PS Blog content, which included announcements. So...let it go.