This is such bad advice.
It assumes that everyone will nail their product on the first go. Almost no-one does.
If I went to customers and pitched Songbox to them with what was the initial version of it I had in my head - I would have gotten nothing. And if I'd given up on it at THAT point then I would have missed out on (a couple years later) having a business that's well into X thousand MRR and even more importantly, working with some amazing people and having truly exciting and unique experiences.
There is no doubt that "sell before you build" will work for some people; but everything and anything will work for SOME people.
For the general populace of makers it is crushingly bad advice.