They have yet to reach production rates where they don't need the other country anymore. The condition was that he showed results, which means they are well on their way, but its not like they have reached national scale yet. Ideally, we kick the garbage out of this country, and subjugate them once we start producing excess herbs.
Just send in a giant dog to step on the kings head. (I considered causing a drought and getting rid of their source of income, but I don't want to upset Hesui)
On a side note, don't we need to check his living quarters for kidnapped/raped women/children?