You must have been reading too quickly, because I said slavery varied on the African continent. However, since many small, rich kingdoms in our world had slaves into the 19th century, I see no reason to assume that Wakanda would be an exception.

As for your belief that kingdoms like Burundi and Rwanda did not have slavery: "Ujiji Ujiji ... was a trading centre for slave and ivory coming from different parts of Lake Tanganyika, including Eastern region of Democratic Republic of Congo, Rwanda and Burundi."

https://whc.unesco.org/en/tentativelists/2095/

As for Karagwe: "Early in the 1840s, Arabs arrived and trade flourished, especially the slave trade."

You can check the citations here: https://en.wikipedia.org/wiki/Karagwe_District

Google does not provide me with anything about Ndorwa and slavery, but it does tell me that Ndorwa is part of Uganda, where slavery flourished. If you have a better source, please share it.