I don't. For one, it's broken on Android (figures floating over text, lines cut in half etc.). But the worst flaw is in the way they made scrolling change the 2D polygon: if you leave the scroll bar in some positions, it no longer actually shows a regular polygon. For a math text that's a deadly sin.
Android aside, the flaw with scrolling caught me initially, but once you get your head around it, it is actually a very intuitive way of illustrating the points being made.
It's not just the 2D polygon, all of the illustrations use the same technique. The gradient descent works especially well.
Yes, the same could be achieved with a slider, but I personally don't think I'd have tried it when scanning through.
I'm not condoning this technique for general use..