So it's supposed to have an effect similar to thorns, where it does damage and poisons the attacking entity? How much damage? Same as the normal attack?

Think like the cave spider, but a bit more potent. It attacks anything that attacks it, and inflicts poison.

Snake venom is still broken, in that a snake can never kill a mob.

In normal minecraft undead mobs and spiders are immune to poison. It'd probably be pretty straightforward to duplicate the poison effect without that check, but it'd be inconsistent with the normal minecraft poison effect.