Yes, every post is a post. A topic is a collection of posts. Informally, the first post in a topic could be called the OP (original post). In the database it’s just post_number: 1.
I agree the terms are a bit confusing. I think that’s because this forum (Meta) is a mixture of developers and non-developers. Developers might be using terms that only make sense if you’re familiar with the Discourse code.
For some context, if you have a JSON formatting extension installed on your browser (for example https://chromewebstore.google.com/detail/json-viewer/gbmdgpbipfallnflgajpaliibnhdgobh) you can easily see the data that Discourse uses to generate the topic’s UI. This topic’s JSON can be seen at https://meta.discourse.org/t/topics-vs-posts-vs-replies/304117.json. It shows that the topic has:
...
id: 304117,
title: "Topics vs Posts vs Replies",
fancy_title: "Topics vs Posts vs Replies",
posts_count: 6,
created_at: "2024-04-16T15:35:47.828Z",
views: 141,
reply_count: 2,
like_count: 12,
...
You can view the JSON without a browser extension. It’s just harder to read.