The question is about how a network monitoring tool works and how to interpret the results it shows. Is this not is scope?
If you were correct, then it would be in scope. However iftop provides host level information about a network interface. This makes it a host monitoring tool much more so than a network monitoring tool. Stated another way, iftop doesn't tell a user about the network, but rather about the status of the host.
Just because something "touches" networking in some way doesn't actually make it a network question, although many people associate things this way. This is not an uncommon occurrence; I can provide plenty of examples along the same lines on sites such as Super User or Server Fault that have the "network" tag applied that are not actually network questions.
A few months ago, someone commented to "mark the question as answered, or answer the question myself." But the question has gone unanswered, so I did neither.
Perfectly acceptable response from you. You even provided a comment responding along the same lines. When this question was again resurfaced because it had no answer, it looks like this prompted the moderator who provided the comment to delete both (i.e. your response meant his comment was no longer helpful and your response would be obsoleted by the deletion of the original comment).
I have a feeling that the only reason this is now "offtopic" is because nobody answered it, and for whatever reason the moderators don't like to have unanswered questions that fester about.
We have plenty of unanswered questions on the site. We don't mark a question as off topic simply because it has been around for awhile and doesn't have an answer.
Questions without answers are "refreshed" by an automated process periodically to try to find a better answer. This often prompts people to look at the question again and sometimes respond in any number of ways (new answer, comments/questions for the OP, edits to question/answers, etc).
In this case, it is far more likely that someone looked at the question with fresh eyes and realized it was actually off topic and should be handled as such. Perhaps they were unfamiliar with iftop specifically and thought it might be on topic based on the reasoning you provided yourself, but then actually had time and/or the inclination this time around to learn more about it and realized it was off topic. Or originally it simply "slipped through the cracks" of the process in place and is now being handled as it should.