This is a lie, and it's exactly the kind of lie that Thiel is trying to call out as insufferable oppression on campus and in intellectual discussion.
Having lived in very 'multicultural' places, and fairly homogeneous places, it's clear to me that there are both advantages and disadvantages to each - and to quash all discussion in one direction is totalitarian and oppressive.
Japan is the most orderly and civilized place on planet earth and it has a lot to do with the homogenous nature of their culture. Nordic countries, though they are 'intellectually liberal' - are culturally extremely conservative and traditional and 'homogenous' in culture and thinking, and this is one of the key ingredients to their social successes, and why simple socialist policies will not reproduce what they have. Sweden has a very popular Monarchy, and official religion (Christianity), a state Church into which 1% of tax revenues goes. Of course they are not very religious at all, but can you imagine a 'state religion' in America? And 1% of taxes going to it? That would be considered 'extreme, extreme right wing' - and yet it is a very real fact in Sweden. It's one piece of the 'underlying base social fabric' there.
I'm not arguing for or against multiculturalism, other than to say there are many drawbacks of multiculturalism, and that in North America especially there is a vicious oppression of ideas and debate in this area, fostered by comments such as the one you made. The issue obviously can be commandeered by populists racists (which is scary), but it never gets that far because someone yells 'racist' and shuts down all intelligent thinking.
I grew up in very Multicultural Toronto and thought it was superior. After living around the world I now live in a very 'Quebecois' part of Montreal that is nearly homogenous - and though I'm definitely a 'minority' and not part of that culture, I like it much better. People here are happy, civil, and there is a sense of community that absolutely does not exist in Toronto or Vancouver (or San Francisco, for that matter).
So enough with the knee-jerk reactionary hyperbole, try to allow for a little more nuance in issues like this.