Lots of companies try and impose things on their employees which a judge would obviously rule to be unlawful. Sometimes they just don’t think through it carefully; other times, it’s a calculated decision that few employees will care enough to actually get the issue in front of a judge in the first place. Especially relevant for something like a non disclosure agreement, where no judge is likely to have the opportunity to declare it unenforceable unless the company tries to enforce it on someone who fights back.

Maybe it's unenforceable, but they can make it very expensive for anyone to find out in more ways than one.