No its not. In fact its completely unrelated because when making book you should care for aspects like font size and text body layout which follow an ergonomic perspective.

E.g. the same booklet layout on a3 wouldn't work at all because the font would be huge. Keep the font size small and you have really long lines...

I regularly print out A4 documents two-to-a-page side-by-side on my office printer. It saves paper, its easier to carry around, and it works just fine. No distortion.

If your docs are US Letter, how well do they print out scaled down two-to-a-page side-by-side?

I do this a lot for printing out (research) articles, and the answer is that the text gets shrunk much more, and the margins increased to account for the difference. (This actually turns out to be useful in practice, since there's much more whitespace to take notes in.)

You are misunderstanding him. He's not saying that you can't shrink A4 down to A3. He's saying that aesthetically you can't do this without (of course) changing the font size. But if you want to retain readability, you may need to keep the font size larger, thus requiring a complete reformatting of the text. That is, you're talking about math and he's talking about the realities in dealing with humans as readers.

Makes it easy for the "large print" edition for people with eyesight issues.