Not convinced that that's what it is. It's certainly a ~3700-year-old statement that the diagonal has length sqrt(2), but there's no evidence that it is, or was intended to be, or demonstrates that anyone had, a proof.

The author of those pages says "It amounts to a dissection of the square on the hypotenuse of an isosceles right triangle into pieces which can be reassembled to make up the two squares on the sides, and I can't see why the figure is exactly what it is if it weren't understood to demonstrate this." but it seems to me that even if all you want to do is write down that the diagonal is sqrt(2) then you'll need at least the square and one diagonal, and adding the other could as well be motivated by love of symmetry as by having noticed that with it there you can dissect-and-reassemble into a 2x1 rectangle or whatever.

Remarkable, none the less.