I think you're just being difficult. All stories of humans v zombies are going to be similar, but it doesn't mean L4D is anything like Dead Rising or any of the other millions of zombie movies and books that have been released over the years.
Source is the engine it's built on, but it's perfectly possible to upgrade that behind the scenes. The entire Source engine got a massive overhaul for HL2:Ep 2 because large outdoor sections weren't really possible before. And any AI script can be rewritten without a serious engine change.