If discussions had a more modern UI with threads or something then the difference might be real. But AFAICT it’s the same set of functionality, so it’s effectively equivalent to a tag.

They sorta do: each comment on a discussion starts a thread you can reply to, unlike on issues where you have to keep quoting each other to track a topic if there’s more than one. It still sucks, especially since long threads are collapsed and thus harder to ctrl-f or link a reply, but it’s something.