Actually, if i read the eventfiles correctly (because i like looking through the code) then it does actually check for that, its just that apparently the AI is very careful as far as that is concerned and wont actually go along with the "cult leader" as it were, in fact you even get the negative opinion from this on top of it, even though you are in the same cult and you "invited" them to begin with. They also dont really switch to being open about it in case you win your independence war that follows either, so you are stuck with essentially vassals who hate your guts for no reason other than the events not firing correctly.
As such its much easier to just go full chaos as fast as you can and conquer your lands directly rather than do the cultist route, even if the latter is less canonical.