That doesn’t mean it isn’t an open standard, that means they are using it as a closed system. This isn’t a case of XMPP not being open, it’s a case of servers using it choosing not to be open. Therefore the problem isn’t XMPP not being open, it’s services themselves not being open. As an example Reddit uses Matrix in their awful chats function, but you can’t message other matrix users there or message reddit users from Matrix. That doesn’t make Matrix not open, it means someone is using it in a way that isn’t open to others.
I don’t know how it is on mbin but on Lemmy the best thing to do is message the admins directly, and not bother reporting. This is because Lemmy’s developers give way to much credit to the legitimacy of forum moderators and do not allow a way for reports to only be sent to administrators, meaning that community mods like these can easily dismiss reports before they are seen by admins. So best thing to do is message them directly at the current time.