When someone comments on a ticket an email gets sent out to the ticket owner (and presumably other participants in the ticket). In keeping with the "email-driven Git workflow" idea, it would make sense for replies to these emails to show up as comments on the ticket page.
Replies to these emails are addressed directly to the ticket owner, so they don't go back into the system.
Aye, I need to set a Reply-to and show the responses on the web. I'm using the ongoing lists.sr.ht work to build out infrastructure for processing incoming emails into the system, this will come after.
I was also thinking about how the ticket owner or comment author's email is visible in the "From" field. Certain people might find that disconcerting. Of course, if things going on in mailing lists is a baked-in feature of the system itself that's just the way things are, in which case it would be good to write that up in your explainers so people know what they're signing up for.
I don't really want to call that out special. It's also by design.