And that's probably the best answer.
"comentar sobre" is to comment about. The thing you are commenting about doesn't need to be present.
"comentar em (no)(na)" is to comment on. You are probably writing your coments directly onto the subject. Like comments on a document or in a blog.
In English to comment on is, in my experience, more often used to mean that they commented on (made a comment about) a thing/topic, than that they left any sort of mark on the thing.
Okay...but that is not what the sentence means, really. «comentar» is not the same as «falar», and it has a more negative connotation
Yes, I understand that, and I agree that at first sight the negative aspect is lost. In English you would write something like:- The couple did not stop making remarks about them. Or, to use argot The couple did not stop dissing them.
It's difficult to make a compact idiomatic translation of the Portuguese.