Deadly Venom deals a small amount of true damage each second for 6s. You can stack it up on enemies to deal increased damage and refresh the duration.
Contaminate deals physical damage to all enemies in range currently affected by Deadly Venom. The damage is increased based on how many stacks of Deadly Venom the enemies have.
So Twitchs E deals magic damage based on the number of stacks of deadly venom. It's the passive (deadly venom) that deals true damage every second, not his E which is a single use active.
Contaminate deals physical damage to all enemies in range currently affected by Deadly Venom. The damage is increased based on how many stacks of Deadly Venom the enemies have.
Hope that helps!