There is a discrepancy between the powers of an admin in open and closed groups.
In closed groups, we can set any individual topic to have comments either open (like most topics), closed (existing comments are shown, but no new ones added) or hidden (no new ones added, and existing ones are not shown).
Open groups have only two of those options, called open and closed (confusingly, this does the same as the “hidden” option in closed groups). Would be nice to have all three options the same in both.