Have they actually said it'll be NDA'd? I know Frogboy long ago mentioned that there would be an invite-only closed test under NDA, but seems plans have been changing a bit since then so I'm not sure if the alpha will be under an NDA or not.
In any case, those under NDA basically can't say anything that hasn't been said already by someone from Stardock. In the latest games, these closed tests weren't actually announced to the public so the selected testers weren't actually allowed to even mentioned that it existed. But if the alpha will be under NDA, obviously that probably won't need to be the case since everyone knows about it.
Pretty much for that reason I doubt that there will be an NDA just for the alpha. SD tends to want to let them stick, as in they don't lift them after the game's release, even. And Yarlen and Zoomba get a little cranky when people break them 