Reddit AmA Goddard says this isn't an accident.
Read on for The Cabin in the Woods spoilers:
"We actually were going to do a downloadable L4D2 expansion pack, where you'd fight in the Cabin world, but then MGM went bankrupt so the delay squashed it," Goddard writes. "But the people at Valve were still cool enough to let us use some of their monsters to fill the cubes in the background (I had a lot of cubes to fill)."
The Left 4 Dead zombies populate some of the monster holding cells shown as the elevator descends into the bowels of the facility. Goddard was excited for the tie-in DLC and is upset it never happened.
"By the way – the game was gonna be amazing. You were gonna be able to play in both the upstairs Cabin in the Woods world and the downstairs "facility" world with all the monsters. Believe me, I HATE all video games based on movies, they always suck, but porting Cabin into Left For Dead felt like the right fit. It pains me that it didn't happen."
Us too, Goddard. Us too.