Dwarf Fortress Creator Explains its Complexity and Origins (Except how is built)

Not into this game myself, but I am terribly curious how the program is architected/written. When I hear the myriad bizarre possibilities, it sounds like it could rapidly become a terrible mountain of spaghtetti code, and with so many strange interactions, would not be naturally stable or playable.

Is the secret sauce that they've made a really well-designed object model that somehow scales up well? Seems like a world rife with terribly O() complexity potentially, too.