Does not always work like that. Did people „want“ cars when there were only horses. Or Facebook when there was just email.
Sometimes you build something and only after people realise that they want built thing.
That's not the right question (as based on the well-known quote).
The right question is: if I showed people a car on actual paved roads, with its load capacity and speed benefits over horses, would they jump for it?
And they did.
But for this thing, they didn't.
This person didn't merely talk about his yet un-made app to some people who couldn't even imagine what could be.
He showcased it, working in full form, and different target audiences could not care less.
There is no single formula for success. Sometimes you are too early and you fail. Snake oil startups are getting billion dollar valuations while a legitimate idea may be rotting in another corner of the world. It's a combination of lot of things. Sometimes I feel many of the yc companies are successful not because people are jumping to buy the license, but because they have a network effect and are vetted by a community and perceive it as the better option while it may not be such. Connections, Money, Advices, Talent, Idea, Marketing, Sales, Management - every single thing is part of the puzzle.
Somtimes you are lucky to get the market fit right of the bat, but many times you have to try hard and struggle a lot before you even have a chance to have a glimpse of market fit. In the case of the author of the blog, if he had build a hype around his product like this is the next biggest thing and got a network effect going (your peer doctors are using it) it may have turned out way different. He wouldn't need to charge it initially or have a free plan that is good enough to get them hooked etc. Don't know there are thousands of variations possible. He simply stopped too early IMO.
Think about the problem horses/cars were solving. It's clear people 'wanted' that problem being solved just by how prevalent horse-as-utility were.