Sorry if this is a dumb question, but could someone please explain to me exactly what the "Commentary" tag means? I thought it meant the image had comments, but it doesn't seem to, unless these are some sort of invisible magical comments only certain people can see.

It means the artist commented on it in the source. We get many of these tags from Danbooru. We don't have the same commentary system that Danbo has so such commentary must be added by quoting it in the comments. Since the tags are added automatically and the commentary isn't the the tag may show up on many posts that don't have it. This has been a known issue for over a decade now.

I suppose you can just ignore it or, if you want, you can change it to commentary_request if the commentary hasn't been ported over or you can port the commentary over yourself.