Ah ha makes sense now! The “Replying to comments” section of that article explains exactly what’s happening. If I understand correctly the community itself (!privacy@lemmy.ml in my above example) is not notified of my reply from Mastodon. If the community did know, then it would broadcast a notification of the activity to whoever else is subscribed to !privacy@lemmy.ml.
Good question! Sorry if this answer is weird :)
For me, I don’t actually interact from Mastodon per se. I wrote a couple of read-only Lemmy & Mastodon clients. One for a weird text editing environment I use (https://lemmy.sdf.org/post/1035382) and via email (https://gts.olowe.co/@o/statuses/01HMQ9N4HQ2ETGZWJS49K5NG5Y). To reply to or create posts, I use a write-only Mastodon client I wrote.
My idea is to exercise the fediverse. In principal I don’t think I should need separate accounts for Lemmy, PeerTube, Mastodon, Kbin, Akkoma, etc.
Right now I’m replying from an account on lemmy.sdf.org as I can’t reply from GoToSocial (Lemmy and GoToSocial don’t work well together right now) and my Mastodon server (hachyderm.io) has a post limit of 500 characters.