Actually, this happens right before the Zombies get unleashed. That eclipse in Der Riese is proof that it was more than just a game, if you didn't notice. When Samantha gets left in the Aether, she creates the apocalypse based on the game she and Eddie were playing just before Richtofen betrays Maxis, which is how the storyline came about. Yes, it was originally a bunch of children playing with some figures, but it became reality when Samantha took control. If you want more proof, play Kino Der Toten again and look for Samantha's room. You'll find the same figurines that are in the ending cutscene.