I don't know what they've proved, but given the sums of money and smart people involved in UI design, I expect they've worked this out. I'm not one to generally trust the authorities, but in this case I don't see any institutional reasons to expect them to get this wrong.
But I'll give it a try on my kindle.
You're confusing the question of whether the UI design people have correctly figured out the population average with the question of whether there is wide individual variation around that average.
It's pretty easy to figure out how you, personally, like to read text on a screen.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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