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.