Sure, Norvig knew how to solve the problem from the start.
But that's another reason why the "TDD Sudoku Fiasco" blogpost isn't a terribly useful source of information (or, from another point of view, why it isn't a great way to persuade a TDD afficionado to change their mind).
Solving Sudoku is blindingly obvious even to mere mortals with even a iota of exposure to computer science and algorithms.