We’re not disallowing it entirely. On occasion, it’s acceptable to answer your own post if you later find the answer yourself, or if you’re answering a moveset post. In those cases is it quite fine.
However, if we find that it is turning into an abuse of the point system or if it is limiting opportunity for others to give an answer, we do step in and remove posts. It is based on discretion, and there will not be a blanket rule like the one you mentioned.
However, people shouldn’t be answering their own posts in RMT if they’re just replying to someone else, or whatnot. Please flag posts like these if you think they should be moved to a comment. I fix these nearly on the daily, but some may slip through the cracks.