Hm, my folding phone doesnt have this aspect ratio and I can still do that.

This would matter for bigger versions of the same thing. For side-by-side, the aspect ratio is unimportant since you naturally unfold to have two of them.

this isn't great design, it's tautology. of course the phone open is twice the size of it closed. it would be hard to make it otherwise.

But apps can just rejigger into widths that fit. It would be more of an issue back when mobile sites were first becoming responsive and we pragmatically chose 2 or 3 media query widths to serve different layouts and then applied the same thinking to mobile apps. Of course the initial mobile apps, particularly on iPhone, stuck to just one and later just a few sizes themselves. As there was a wider range we did things more responsively on a continuum.

I still think the broader point of there being design reasons to hit these ratios stands and is great. I just think the idea that it needed to be precisely twice the width so apps can fit full-sized doesn't hold too much because there isn't really a "full size" notion. Or if there is, the full-size is probably the double width one. If it wasn't exactly twice the width, most apps are already built to fit into varying iPhone/mobile screen widths.

Apps already work on Apple devices with many different aspect ratios.