I had never heard multiculturalism mentioned in reference to Thiel (I'm no expert on him), but apparently he co-wrote a book called "The Diversity Myth: Multiculturalism and the Politics of Intolerance at Stanford". Looks like there were 1996 and 1998 editions with slightly different titles. [0], [1]

[0] http://www.amazon.com/Diversity-Myth-Multiculturalism-Politi... [1] http://www.amazon.com/Diversity-Myth-David-Sacks/dp/09459997...