Well, the title says you play the game by modifying the game's source. Turns out you don't modify the game's source, you modify specific snippets of code that are provided on the page.

Last weekend I hacked around in the source of various online games, I anticipated this to be a similar experience but was disappointed to find that we weren't supposed to actually change the game's source.

Well you can play it that way too, if you want. Try overriding some of the internal functions using creative methods. :)