Good point. Very good point indeed.
You could add “Reply” to that list, which is the term discourse uses for “comments”. Indeed, “reply” is probably even a slight bit less ambiguous because discourse also has comments (when some replies in topic are displayed under a blog post).
I wouldn’t say “post” is ambiguous (it’s pretty clear what a post is and what isn’t), rather: it is broader than, e.g. “reply”. But that is a good thing because it’s useful to have a term that includes both OPs and replies.
So what really remains as a point of contestation is the term topic and I don’t think it is really that ambiguous, especially when the forun also uses the term category, which makes it pretty clear that topic doesn’t mean category. So it means thr34d. Problem solved.
Besides, you have to admit that as much as “thr34d” is an established term in the online world, “topic” is clearly less technical and more common sensical, no? Just think of a person who has not spent 19 years working with forums when they look the button “New Topic” vs “New thr34d”.