It just seems to be how Discourse works.
If you move the post from a one-post topic to another topic, Discourse doesn’t immediately delete the one-post topic. Instead, it closes it and sets it to be deleted within seven days. During that period the post in question appears on the forum twice: once in the soon-to-be-deleted one-post topic, and once where you really want it to be. It adds a note that the post has been merged, and a link to the other topic (this is what causes the back link).
I unlisted the one-post topic (instead of deleting it) so as not to interfere with Discourse’s normal operation. I wasn’t sure why it hadn’t been deleted, or what unintended consequence there might be if I were to delete it.
Do you know why Discourse does what it does? That would help me answer your question.