It’s a counterexample to a single step of reasoning(large multiverse of people --> God), it doesn’t have to be globally a valid theory of reality. And clearly the existence of an imaginable multiverse satisfying a certain property makes it more plausible that our actual multiverse might satisfy the same property. (As an analogy, consider math, where you might want an object satisfying properties A and B. Constructing an object with property A makes it more plausible that you might eventually construct one with both properties)
It’s a counterexample to a single step of reasoning(large multiverse of people --> God), it doesn’t have to be globally a valid theory of reality. And clearly the existence of an imaginable multiverse satisfying a certain property makes it more plausible that our actual multiverse might satisfy the same property. (As an analogy, consider math, where you might want an object satisfying properties A and B. Constructing an object with property A makes it more plausible that you might eventually construct one with both properties)