the author basically says "I tried to do X, and just got stuck into the code. I failed. I started again, designing the entire User interface. After I was done designing I started the coding. This worked perfectly. So do the Design first and then code."

Maybe the reason that he succeeded the second time around was that he had already tried and failed, and so knew where the pitfalls were. It's not proven that design on it's own, without trying out some ideas first, is going to help. Many people would say that it' doesn't. I'd say that big upfront design without any reality checks can be a major hindrance later on when it turns out to be impractical.

Also "designing the entire User interface" ? Is he talking about graphic design or program architecture?