It gives the illusion of security, but they would absolutely not deter a determined threat actor.
The only reason that the water supply isn't poisoned is it's unpractical for a single person to conduct the whole exploit chain: Construct the poison in enough quantities, gain access to facilities supplying the water, and actually throwing the compound in it. It's unpractical even for "underground" types. Especially the quantities required.
Mathematics and computer science is a different story in my opinion. You cannot restrict science or thought. You can try, but good luck. The most you can do is delay it. If there is an attack that enables someone to flip a Tesla on the road (as suggested below), the security theater will hide the attack from common folk, but determined actors will reach it eventually, and at that point, they can deploy it as they wish. And in contrast to the water plant, the logistical endeavor to exploit it is absolutely easy in comparison: slap a sticker on your vehicle.
Security by obscurity or by theater is rarely a good strategy in my opinion. We should absolutely be transparent about these kind of things, and allow researchers full access to develop attacks against these systems, and effectively communicate when they are found.