That's the one plus I've had on the closed screen Fold, that it's actually usable one handed, and I have fairly large hands. The typing does stink, there is no way to get around that unless they can improve predictability of the keyboard. Sliding letter to letter works perfectly, but hunt and peck I get some errors that are annoying. But keep in mind we are comparing this largely to the Max iPhones, the screen size on the Fold's is pretty much the same width as the non Max iPhones. I'm holding my Fold 2 next to my wife's non Max iPhone and don't think there is more than a mm or 2 difference. I make just as many mistakes on her phone that I do on the Fold's outer screen.
But for swiping all the way across, one handed typing, and reaching all the way across the screen with my finger I would have to say the Fold when closed is a dream to use. No more balancing the phone just to get my thumb across the screen, especially with Apple's horrible and disjointed system of navigating back.