For reference:
24. Advertisements (including signatures within the post area) in posts aren't allowed unless the post is in a thread you started and is really substantial and useful.
I think there should be no ads unless you are advertising something directly related to the thread's subject.
I don't see anything from your reference that says the ad should be related to the thread. I think "Substantial and Useful" refers to the content of the post and not the ad.