Malcolm X argues that they are not Americans because I'm guessing that...
1. African Americans were not treated like Americans (whites) on the basis of their skin color, so why call them Americans when they aren't treated like so?
2. African history doesn't just start at slavery. It began years before, therefore their roots are deeper than the country they were born in, it goes back to their ancestors in Africa and their culture derives from that, not the U.S culture.
3. The only factor that made the slaves American was due to slave trade; they were not born in the U.S, just brought there to help profit the whites' and produce mass amounts of cotton. But by the time he was born, most slaves had American-born children so I may have contradicted myself. I hope I helped.