They want coordinated action to prevent a multipolar trap and potential catastrophe by future superintelligent AI (which may emerge quite rapidly if RSI ever hits an inflection point). It's not rocket science. The rules would apply to everyone. The rules they're advocating for are not "only OpenAI and Anthropic can make AI".
Critics of Yudkowsky have been trying to misportray that for years to make him seem delusional. You can see an example of that here: https://www.tracingwoodgrains.com/p/reliable-sources-how-wik...
(Roko himself is genuinely crazy, but largely because he became a neo-Nazi.)
If you think ASI is plausible this century, you would have no reason to not fear the end times yourself. If you don't think it's plausible it'll be created this century or perhaps ever, fair enough, but I'd bet against that.