If splitting the UI is useful then it feels like you should just let me choose to split it even on a single display rather than deciding for me. My laptop has a display aspect similar to a folding phone when unfolded, so if splitting each half of the display into two panes is useful it should be useful regardless of device.
Personally if I bought a folding phone, I'm probably buying it specifically because I want a bigger display for existing content. I don't want your app to decide for me, to treat it like 2 displays. Imagine in vi or emacs auto split panes based on your aspect as shown in the article. No please! Let me choose when I want it split
It would be good if there was an OS level setting for something like prefer-unified-display in the same way we have things like prefer-reduced-motion. This should be a choice.
There is a key difference between your laptop and your phone, in that you are usually holding your phone. I imagine UX tweeks for wide devices in the hand that could be useful, but mainly for games and such. Keeping controls to the two sides for if held in both hangs, rather than at the bottom for a more standard portrait screen, and so forth, but these things have already been done.
Other changes for the screen getting wider I agree would be best done for all devices. If you don't want you main layout to go beyond a certain width, maybe jump to two columns, or move footnotes up to the otherwise empty space beside the text that refers to them, or images that would be under a paragraph to its side, …
Just letting the user choose would be my preference. I have a large main monitor (32", with a 24" 1080x1920 portrait screen to the side) given how close my face is to it much of the time, and while it is sometimes (two document pages side-by-side, games, videos) used as one large screen most of the time I use it as a flexible multi-screen (50/50 for two apps, or 33/66 or 66/33, or occasionally split three ways, sometimes split all over the shop like a Mondrian painting for a full-screen terminal session). I feel it would be nice if a phone UX allowed at least two apps side-by-side, or single apps allowed me to split views that way sometumes and sometimes not.
I usually want the desktop site and zoom that the mobile specific version.
The new Duo exhibits Apple Mouse scale UX oversights.
It's sad no one bothered to copy ideas from Windows Phone 8, its UI had a lot of thought put into it and was exactly as efficient to use for left-handed users.
To support a left handed person it should fold open to the right, also that way it’s a bit weird, although I’m curious to see how the animations would work for notes apps for Arabic language UIs where text is written from right to left.