Beyond racism some of these things happen a lot in startups. As a black person as well, i might have experienced this in a much lower stakes, for example, hired at a startup.

https://news.ycombinator.com/item?id=22091777 The behaviour sounds totally unacceptable. "Is there a way to do this without making it painfully apparent to him that he's being diluted to 10%?" Zuck.