But what does it mean to say "in a world where 0+ strings occur more often"? Typically that's not some immutable fact of nature. It might be merely an artifact

But what does it mean to say "in a world where 0+ strings occur more often"? Typically that's not some immutable fact of nature. It might be merely an artifact of the current state of the world, that could change next year. Eg: cigarette smoking was extremely common and a recommended social activity in the past, but now it's not. If you asked someone would they still smoke if cigarettes were harmful, and they tell you that it's an out of distribution question because they don't know a world were cigarettes are harmful, you'd question their intelligence. We want predictive models to be able to estimate the consequences of different (counter-factual) actions to then pick the best one.