I'm not denying that he's good, and I'm not denying that he can probably hold a lot of broad structure and design in his noggin, but I said "the entirety of an application" meaning every single detail of every single class/object/method. That's what I say is impossible.
In my experience, as soon as you move implementation details out of your head to concentrate on other structure or code, it becomes extremely difficult to keep track of the scope and references of said details, and you start hitting problems when you refine bits of structure and forget that you've used the same structure in multiple places.
I know my limit is about 1000 lines of code, give or take (my largest application is a flash game at roughly 2000 lines, the longest segment in one place is just over 1400, and I don't even look at a large chunk of it any more because I don't need to (actually, right now, there's something wrong with it, but I have no idea what because I didn't touch it and it broke (actually, it's an issue with a class that should be importing with the creation of a movieclip, but it's not importing for some reason, that area of code just happens to be where it occurs (or more accurately, doesn't occur)))).