I find the NDA strange. If they're looking to do a larger playtest, then they should do that privately. They have playtest rooms at WotC headquarters, and can invite people in to sign NDA's with far less work than trying to run at a con and taking up table space.
The only reason to do it at a convention would be to start showing off what they're working on publicly.
All I can think of is that they want people to be seen playing the game, and once they're done, to go out and tell people it was fun and on the right track, all without divulging details.
It's probably just to get more posts out there saying, "I've played it, I can't talk about it, but it's good."
Or, the designers are simply anxious to see people playing.