From her tumblr:
It doesn’t matter if any of Magnus’ background is white. Casting him white would erase that part of his background that is Asian. And important. There are plenty of roles out there for white actors. Most roles are for white actors. This is not one of them. There is very little I have control over as regards casting. I cannot pick an actor for Magnus. I don’t have that ability. But I can say, and say strongly, that I want them to cast an Asian or half-Asian actor, and I did. It is pretty much the one ironclad demand as regards casting that I have made, i.e. : if you don’t cast an Asian actor, I’ll never talk about this movie again, nor will I see it.