Sorry, sometimes I say “post” when it’s “topic”, because a reply inside a topic is already called a “message” I believe, so calling that a post would be weird to me. And I guess we got so used to the term “post” with social media, that to me everything is a “post”.
So yes, a topic. The big “bucket” of all messages, like this very same topic we are now using to discuss this.
So, the number (ID) in the URL is what matters here. Which is good to know. Unless, as you said, your post (your message) was moved to another topic. I mean, the person would still see the second message, because of the /2 but it wouldn’t be your message.