That particular (old) setting is not the greatest, and can essentially be considered deprecated unless you want to depend on its behavior. It only applies to TL0, and as you noticed it does not include oneboxes.
There is another setting that is more powerful and probably does what you want, min trust to post links. It allows you to choose the trust level that the links applies to, and it counts oneboxes as links.
As flawed as it may be, it is not really replaced by min trust to post links, is it? It makes sense to allow TL0 users to post a few links but not too many…