The story
This name for sorghum is probably connected with both the distinction of Muslim versus non-Muslim and the distinction of sub-Saharan Black race versus other race; the history of usage of the word kaffir includes derogation, depending on time and place. For more information, see kaffir cabbage and see the Wikipedia article on kaffir as a racial term.
Etymology from Wiktionary, CC BY-SA 4.0.