They do use outside staff for development, at least in some cases, so perhaps talking about the 400 employees is a little misleading. For example, Hidden Path E

They do use outside staff for development, at least in some cases, so perhaps talking about the 400 employees is a little misleading. For example, Hidden Path Entertainment did a lot of work on the new CS title, and Wikipedia tells me they also did some models work on Left 4 Dead 2. I've no idea about the other games, perhaps they are all done 100% in-house.