I'd disagree with that one data point proving too much. For a start, I don't think it contradicts the "Zuckerberg-ness" issue- graduates from Waterloo are also very likely to be hoodie-wearing white men, I don't think that being Canadian counts as particularly 'diverse'.

Sure, PG is open to graduates that aren't from Ivy League universities. That's one factor. There are a lot of others.