"Frankly, I don't see a point in pre-selling, do you?" If you have ever developed something that no one (or too few) people wanted to pay for you may get more cautious about developing something without assessing customer buying interest first.
You are very unlikely to get it right based on a hunch without multiple iterations with real customers. That means that you will end up iterating whether you build it or not. So its best to first iterate without building: you get more iterations and less wasted development that way.
As the interview explores, that doesn't mean building nothing, but it does mean validating an idea first and foremost.
I understand that this can be frustrating for a developer. We tend to think, 'if we build it, they will come' and we don't like sales. But personally I've wasted too much time building elegant software... nobody much wanted, to build new stuff without validating it first.
If you "don't get" this interview, I would suggest a second listen. It contains a critical lesson.
I realize this approach will probably be hated on by the HN community, but for a certain set of products, it makes perfect sense.
I'll add one more thing:if you guys were selling hardware, you'd have a much greater appreciation for this technique!