Personally, I find the feature useful to comment in any discussion, not specifically to reply to another users comment.
Discussions in d:Wikidata:Property_proposal/Overview are mostly un-indented (or prefixed by "*") .. would be nice have a way to use it there too.
Interestingly, it works on d:Wikidata:Requests_for_deletions, as one can just reply to the original comment. (Maybe we should do away with the section header on property proposals to make it work there).
That might work. phab:T265750 (manual adjustment) would also give you some more options.
