My question is what are limits for such a superuser?
There aren't any. As long as your answers are consistently judged by the community (via votes) to be of high quality, no rate or post limits are set for users.
Low reputation users have timed post limits (see The Complete Rate-Limiting Guide for reference), and if answers are consistently judged as poor ...
Right at the bottom of the Participate in meta privilege description you've linked:
Because we allow anonymous participation, we require a small bit of parent site reputation to prevent spam, and ensure that meta is for active, engaged members of the community.
That +15 is caused by three upvotes (+30 reputation) minus 15 because of the unaccept. Since all events happened on the same post, they're lumped together.
I do see the unaccept in your reputation history: