[1] https://news.ycombinator.com/item?id=5427317
It is not uncommon to have such NDAs especially for senior-level positions. But now they have started coming for pure bureaucracy.
I have heard that some big-name companies do not even let you tell to the outside world that you interviewed with the company. This would happen when say, just for example, if a company like Facebook would have positions for cell-phone design experts. The justification given is actually reasonable -- the media picks up on such moves by companies extremely quickly.
My bet though, it is almost never about any "real awesomeness" of the project. Those projects are most likely happening at several other places in the world.
a) this might unfairly restrict your future job choices or personal development activities.
b) It is a classic a*hole indicator - so a good warning about the sorts of people you would be interviewing with.
<edit> Using the word "awesomeness" is another good indicator</edit>
However, it may not even be a legal barrier, there are some companies who won't hire or engage conflicted individuals due to risk of litigation.
Definitely consult a lawyer in the appropriate jurisdiction though.
I also would be leery of any job posting that wouldn't give some reasonable details. I get a stealth mode start-up, kind of, although I don't totally agree. But that doesn't mean they can't say who they are and at least what space and technologies they are in/wanting. I see neither of those as competitive advantages that need to be hidden.