The NDA is not a valid contract because there is no consideration. For a contract to be valid each party has to gain something. This is why many contracts include a token consideration of $1. This one didn't, so it's invalid.
What about NDAs for interviews? What am I gaining (a chance at getting a job doesn't seem like a gain)?
A chance for employment (over an outright dismissal) is a recognizable gain.
You are however, free to decline with the appropriate consequences.