When talking about third party handheld games becoming less common, I don't think people usually are referring only to volume, but also the general status of the titles being released.
It's kind of like how the Vita has a ton of games, but almost no big hitters.
Obviously the 3DS has a notable line-up of software big hitters, and will continue to dominate the traditional Japanese market, but the main conception would be "Where is the next wave from third parties, which third parties are and aren't present, and how does their high profile and low profile output compare generation over generation in terms of volume?"
Generally speaking Sony doesn't run that kind of NDA. If someone wants to wait for the event because they think it gets them more coverage, they might, but otherwise there's no signs they're blocking anything.
