So I'm listening to the interview. And it seems that Niel is saying that long ago, when the planet formed, lighter density rocks floated to the surface.
Which, of course, is true. But this means that there would be an essentially even distribution of granitic rock across the surface of the earth. The earth certainly does not have that today, with mountainous regions having a crust at least twice as thick as that of continental crust on average (itself thicker on average than oceanic crust).
Now why would that be? How could that happen? Oh, gee, I don't know, maybe collision? Since granitic rock doesn't really subduct, it crumples and a divergent zone is produced elsewhere. This produces oceanic crust.
What's so difficult about that? Granted, I have quite the simple model, but still.