Perhaps and that's how it should be. What isn't clear is whether the victory is determined by Zed vs Survivor or Survivor vs Survivor.
Its a common mechanic. Look at any multiplayer co-op game.
Assault the _. Switched. Defend the _.
You are now playing a terrorist. Switched. You are now playing a counter terrorist.
You are playing a survivor. Switched. You are now playing as infected. Etc