19 is right and undersold, and the reason is that everyone testing foldables tests them alone.
The half-open posture is not a layout, it is a social arrangement. Two people, one device, facing each other, and neither has to hand over their phone. That solves an actual awkwardness, handing your unlocked phone to a stranger to read something is a small unpleasant thing that happens constantly when travelling.
So the product is not "an app that uses tabletop mode". It is the situations where handing over a phone is the current answer: a menu translation, a price negotiation, showing a doctor a symptom description in a language you do not share. Same device, two readable sides, nobody holding anybody's unlocked phone.
Market is small. Agreed. But you have written it down as a layout and I think it is a use case.