"If you unfold a rectangle, you're going to get something that looks squarish"
If the folding technology works and can be applied to a tight enough curve (and hence to a narrow enough device), you could have multiple folds. for example a device with 6:9 ratio with a fold at each end would fold out to 18:9 tablet. With phones nowadays getting thinner than 7mm, the fully folded device would be less than 2cm thick, just like old dumbphones and, incidentally, much more comfortable to hold in the hand for use as a phone.
The bigger tech question isn't so much as to how it can fold as to whether it manages to consistently unfold to a totally flat position without leaving a very slight angle