The problem is, Extreme Programming is promoted as the approach to use when you don't know what your final result will be like. "Embrace Change!" As far as I understand, Jeffries was not being stupid in that series of posts. He could have looked up the right algorithms at any time, like you or me. He was just trying to make an honest showcase for his own methodology which says you're supposed to be comfortable not knowing exactly where you're going. It was an experiment worth trying, and if it worked it would've helped convince me that TDD is widely useful. Like that famous presentation where Ocaml's type system catches an infinite loop.

The main difference I see between those is that Norvig knew how to solve Sudoku problems before he started writing a program, while Jeffries didn't

When you already know exactly how to do something, you've already written the program. After that, you're transliterating the program. The real difficulty in any coding is figuring out how to solve the problem. In some cases, it's appropriate to start writing code as a part of the process of learning how to solve the problem, and in those cases, writing tests first is not going to be especially useful, since you're not sure exactly what the output should be, but it certainly is going to slow down the programming quite a lot.

So, I'll agree that Jeffries should have understood the problem space before writing many tests, but not that understanding the problem space is entirely a pre-coding activity.