No, I'm saying that his argument is nonsensical. I can go well past the edges of the iPhone with my thumb, probably by another 33%. I'm sure most people can, my fingers are quite average length (much to the disappointment of my wife).
I could probably extend well past the edges of the Android phone, as well. So I don't think that "average thumb length" had anything to do with the size of the iPhone screen.
I've got pretty average-sized hands and finger reach (which I know from piano lessons). With an iPhone 4 firmly gripped in what I consider a natural way, my thumb can reach comfortably to the top-right corner of the iPhone display and very little further.
So, while I think the diagram is exaggerated, the writer does have a point. My guess is that the 3.5" screen works very well a very large percentage of adult hands (say 75%) and acceptably for, say, 95%. I think a 4" display would probably pull things out of the bell curve. I suspect given that Apple probably made a LOT of prototypes that the iPhone's screen size was determined empirically.
To me, the iPhone4 feels almost exactly right in terms of width (I think it could be up to half as thick without feeling flimsy).
That's a fair observation, but you must admit it takes more effort to tap with a thumb fully extended like that. Especially taking precision into account, since you end up tapping with the flat part.
I think you got his argument backwards.