Very good questions! I'll try to explain my rationale...
First, some definitions. In Discourse, there are no threads. Instead there are topics, and a topic is a collection of posts. A reply is a response to post.
In my ideal world, I would have preferred a new topic and either @mention the key people from the old topics or if there is only one old topic, reply to a post in the old topic and point people to the new topic. Generally, when I see a post that is a hijack or off-topic, I tend to split off that post and any replies into a new topic.
Ordinarily, I would have kept your new topic in favor of your reply in the old topic, but because your reply sat for a few days, it likely had notified people on that topic, so I deleted the new topic with the idea that maybe I'd come back and split off any conversation that happened on the old topic later.
But now that we are talking about it, maybe that wasn't the best option? Now that you have had a reply, I've undeleted your new topic and moved your post on the old topic (and the reply) there.
I dunno. I'm just trying to dedupe conversations, but keep them somewhat organized as they develop. It's more art than science. I'd love any feedback about what I should have done differently!