This whole chain of posts is a topic. Every each message is a post. I surely make a reply, that is a post, to you, but if I would use reply button it would be connect as an answer to one of posts.
Same thing in Facebook where everything under original post are a topic. When someone is posting direcrtly under original one, like staying at root-level, it is just another post. If one answer to someone and create new thread (what Discourse doesn’t have because thread is basically only a filter showing just part of a topic) that is an answer, and a post too.
So a post is a reply, technically, if someone uses reply-button. This, and if I would mention you, is of course semantically answers, but not technically 
Sure. Coders and devs want everything beeing in logical order and behind exact definition. But at same time they have horrible and and even weak imaginary — or as a code wizard I know is formulating same: they are slaves of end users and incompetens bosses 