Enforceable is a level of risk that we can't decide for the OP. If there is any chance of enforcement that is a chance of them taking credit for OPs work, or other financial issues. Not to mention lawyers are expensive, even if it's not enforceable, is OP willing to pay for legal fees to prove it?
Personally, even if it's not enforceable and they're completely morally justified the question remains, why open yourself up to any risk at all by using company equipment when you can literally use any other computer to do the work?