The original DDOI subreddit specified that it was one way in the rules (at least at first, may have changed) , therefore it made sense to have a community for the inverse. If the small community on this small platform doesn’t have such a rule then IMO they aren’t different enough to warrant separate communities.
I actually had to look it up as I couldn’t remember why I made the first switch. PolyMC was forked from MultiMC after they dropped third-party modpack support. Then there was some drama with one of the devs of PolyMC, spawning Prism Launcher