Men such as Bill Gates did not impoverish Africa, and his foundation does substantial work in Africa, described at https://www.gatesfoundation.org/where-we-work/africa-office .
How about the rest of the billionaires? Wikidata only contains about 500 of the Oxfam article's 2000+ billionaires. Including Bill, less than 20 of the 500 Wikidata billionares are listed as philanthropists. Bill may be the only of the listed billionaires that's documented to be funding work in Africa.
Wikidata query: shorturl.at/cknyW