The way I understand, it worked because alternatives to the ozone-destroying chemicals were known to be possible, and the costs of getting manufacturers to switch, as well as further R&D, weren't that big. I bucket it as a particularly high-profile example of the same class as most other international treaties: agreements that aren't too painful to follow.
Now in contrast to that, climate agreements are extremely painful to follow - and right now countries choose to make a fake effort without actually following. With a hypothetical AI agreement, the potential upsides of going full-steam ahead are significant, there are no known non-dangerous alternatives, so it won't be followed unless it comes with hard, painful consequences. Both climate change and AI risk are more similar to nuclear proliferation issue.
Montreal Protocol have worked despite no threats of violence, on a not entirely dissimilar problem. Tho I share the skepticism on solution to alignment problem.