We're still talking about a new company (first game) new engine (only graphic demo so far now) and not even a playable demo/trial.
Rather say "just 800 pre-orders" I would point 727 out of 800 are linux user who trust in the project and just 81 out of 800 windows user.
And don't forget we're talking about the "so large number of windows user around the world".
Probably there about 3000 or more windows user around there which were used to pay 20~30$ for a card game "solitary 2.0" and don't give a ** about uningine.
Comparison with battlefield/modern welfare are misleading since we're talking about real time strategy.
The last bestseller we can make comparison with would be StarCraft II, but I don't see why 1million of windows user who bought it wouldn't be interested in OilRush. (I mean: 87? really?)