There are methods to enforce such things, companies are made up of people and people are vulnerable. Iran has had a hard time to keep their nuclear physicists alive, they encountered remote controlled sniper rifles on their way to loved ones from time to time. I expect the same to happen once nation states and the relevant security services identify this as an existential threat. They might react too slowly, but if they don't then nothing is off the table. At the moment the number of people that are actually capable of training such large models isn't large, of course this will change over time and I agree that it is much harder to control than nuclear proliferation, but datacenters and public companies are very vulnerable to sabotage and assassination.

My hope is that we collectively realise that it isn't in our best interest to create something that would have the capability to replace us, if something goes wrong. A bit like we have been capable of creating human clones, but we haven't made use of that capability.