By 1910 universal compulsory education was already in full swing in many places. Take Massachusetts, for example. If you examine the textbooks being used in Massachusetts at that time you'll see the same phenomenon -- they're much more advanced than modern textbooks. And they were not just for the rich, Massachusetts already had compulsory education.

And by the same argument: where are today's equivalent textbooks for the rich? Our wealth gap is as big as ever, yet I see no books even for the rich that are so advanced.

I'd be curious to see an example.

I seem to recall a 19th-C. MIT entrance exam as comparable to or maybe behind what my better high-school classmates learned.

But he's doing it across a couple generations. Our material wealth is so much greater that it's reasonable to ask whether our level of education has kept up.