I thought it is not allowed.
Don't post threads just to share your referral link, especially when the site already has a thread. If the thread is interesting, unique, or otherwise helpful, or the site doesn't have a thread already then it's okay.
From the quote. They state "Don't post
threads". What about
post in the
official thread which already
have a non-referral link?
Hard to say without seeing the post, but no, that would be spam.
Posts with referral links can be okay if they are on topic and substantial. For example, if someone is asking what the best exchange is, and you post a decent reply, that also has a referral link that's okay.
Essentially, if the purpose of your post is to share your referral link, then don't.
I've seen some take this literally and make a post essentially saying check the referral link in my sig. Just because it isn't directly in your post does not make it okay.