To be fair, he raises a fair point. The title should be "don't invest heavily in coding from the start, talk to your future customers first, if those are available, if you are trying to make money with your project" but it's less catchy. I've been guilty of building things nobody needed and it's not a nice feeling

He's quite clearly talking about getting paying customers before building anything. I don't think this about design docs before prototypes where you're more an employee not an entrepreneur.