I think the closest analogue is the Jabri (although I agree with MindofShadow that they have to come back sooner rather than later). Basically, a sometimes antagonist, sometimes ally. I don't think allowing them to remain is so hard to believe. The Jabari have been allowed to remain, for example. Additionally, they remain as part of a specific political deal that T'Challa agreed to and stopping them would be breaking his word. I'm not saying he is incapable of doing that if the security of Wakanda is at stake, but I would argue he values his honor and keeping his word when possible.