I can't access the article but it sounds bullshit to me; although the portuguese didn't mind at all being super racist towards their colonies. Guinea comes from portuguese Guiné, which has (so far) an unknown origin but historically described a long portion of sub-saharan costal land in Africa. In the region there used to be local powers called that because it would mean "warrior chiefs" or something in a local language. It's not exactly known if they were from Mali (whose city of Djenné traded with portuguese folks) or from Gana (another name related to the story and with uncertain origin).