Ed Beech said something about a Religious Victory requires you to found the Religion, hence the reason why Kongo cannot compete for it. One could quite easily infer that maintaining control of a religious Holy City is not a requirement to win a Religious Victory. The only thing that is required is that you have founded a Religion that is dominant in all surviving Civ's. Although not conclusive proof, i think it is pretty safe to assume that maintaining control of a Holy City is not required to win.
I'd think that if someone come close to win a religious victory you could stop them by conquering their holy city, however you will not win a religious victory in their place.
IMHO that's just as likely, and would make more sense.