It may be that Rwanda and Burundi and their earlier conjoined names are part of East Africa because they were once part of German East Africa, which basically became Tanganyika (no coins) and then Tanzania.
You're correct that what's now Rwanda and Burundi were part of German East Africa, up until WWI when they became a Belgian territory. However German East Africa is a separate country in the Numista list. The country East Africa on Numista covers the currencies which circulated in the British colonies
Trying to get a coin from each UN country with circulating coins. 20 to go!