What exactly does it say that makes you think you would not be able to work for someone else in the same area?
It sounds like you're talking more about a non compete agreement than a non disclosure agreement. A lot of companies will have employees sign them saying they won't work in the same industry for X amount of time.
Since you never did any work for this employer, I don't imagine you have any of their "trade secrets" to disclose to anyone else.
I am not a lawyer, but I doubt it is enforceable even if you did sign something saying you would not work for others in the same industry. My understanding is that in order to have a valid contract, both sides must have consideration (something of value). Since you did not do the work, I don't think they can enforce a contract for you not to work for others.
In the future, I would read things carefully before signing them and don't be afraid to push back. Many companies have boilerplate contracts that don't appreciate the concerns a freelancer has compared to an employee. I've been handed contracts that would basically have put me out of business had I signed them. Things like "You won't do business with anyone we have a relationship with." That is not only not reasonable, but how would I know who they "have a relationship with" in the first place?
So far I have never lost business for questioning a contract or saying I'm unable to agree with certain things in it.