Right, I don't think it's 100% accurate either. It gives just some hints what might be related, but like you said, it's not necessary the best possible measure of similarity.
Jaccard similarity does not count only the number of people who posted to A and B, it checks how many people posted to A, how many people posted to B, and how many of those people posted TOGETHER to A and B. That togetherness gives us hints what is related, and after it is computed, we can divide by the total number of poster to both A and B (independently), which brings the value to something that we can use to compare against other subreddits. If that value is close to 1, it means that almost all users who posted to A have also posted to B. If it is close to 0, then the overlap is much smaller.
Jaccard similarity does not count only the number of people who posted to A and B, it checks how many people posted to A, how many people posted to B, and how many of those people posted TOGETHER to A and B. That togetherness gives us hints what is related, and after it is computed, we can divide by the total number of poster to both A and B (independently), which brings the value to something that we can use to compare against other subreddits. If that value is close to 1, it means that almost all users who posted to A have also posted to B. If it is close to 0, then the overlap is much smaller.