Minor Civs "stealing" technology, but not Barbarians

[CivWarGamer]

Are the two not tied to the same mechanism?

 

[Pazyryk]

I'm bumping this thread because I don't understand this mechanism at all, for city states or barbs. Any explanation would be helpful.

I need to restrict city state techs (so I can specialize them) and perhaps barbs too. Does anyone know if the existing GameEvents (PlayerCanResearch and PlayerCanEverReseearch) will do this? I guess I could SetHasTech every turn for every city state, but that seems rather extreme.