How does the Pyromancer achievement work?
To this day, it appears to still not be working from around early November 2011. Even looking at the achievement through Steam, as suggested here, didn't show the numbers. It is, however, possible to see the progress tracked through the game's HUD.
While in game: menu -> view achievements, and there will be a box you can tick that says show in HUD next to the pyromancer achievement.
I can personally say I've gotten Pyromancer in the past year, past November 2011, and it does show mine at 1,000,000 while in game.