"America's first choice in post nuclear simulation" is just clever advertising for the game. This does not apply to the storyline.
This is how I see it. Isn't Fallout 3 a video game, Therefore a simulation? In the Fallout universe it could all be real, Not a simulation within the game. "America's first choice in post nuclear simulation" Doesn't necessarily apply to the story within the game, Its like Crazy Taxi which was advertised as "Our training manual (Video game)" Or something like that. Clearly, People aren't driving off rooftops in taxis and landing perfectly fine. Its just something to sell the game.