[thirsteh]

He wrote PuTTY and all of the assorted tools. I'd say that's a fairly large codebase.

Also, one of the main reasons Go was created was that the authors were tired of the compile times caused largely by preprocessing.

[reirob]

And he wrote a Collection of Puzzle games that work across different operating systems including Android: http://www.chiark.greenend.org.uk/~sgtatham/puzzles/

I would say Simon Tatham definitely has good experience in programming.