The test worked! Now I’m replying to my own email, also via email.
OK, this reply from email didn’t thread correctly with Testing whether creating a topic via email works. The problem is likely mine. Since I hadn’t configured my email notifications according to this use case, I didn’t receive an email notification about the first post, and silly me I replied to the email in my Sent folder.
Of course that didn’t work here. But still, a moderator could manually append this topic to the parent one, and a user mistake could still fixed in the “archive”.
(I’m posting this reply via web.)
Now that my preferences are set up correctly, I did get an email about my own reply. To which I am replying now via email. This should create a reply to the existing topic, and not a new topic.