Except that this is a lie. In the US, it is ILLEGAL to have an employee agreement that states this (at least if you are an employee potentially capable of unionizing).

If I wanted to make up an untrue reason for keeping the data confidential, I would make up something more interesting than this. But instead, I'd prefer to simply keep mum. Or not... if I've been particularly well-compensated in my previous position I might be quite willing to share that salary information, accompanied with an assurance that I would be willing to negotiate a lower salary (if I am actually willing) in exchange for other things like flexible working conditions, autonomy, or greater responsibility.