I know the m&m does not mention the poison immunity. But I think it is a typo, all undead should be immune to poison damage as a rule. Poison is something which is harmful to living things, it kills them by disrupting their internal workings or something like that. Corrosive things that hurt and melt matter are acid. And acid and poison damage is seperate in the game. Thus, death fog spell works well on liches. They are far too slow to walk out of it too. But I don't think a lich, zombie or skeleton should be hurt by a poisonous snake bite. All undead must also be immune to skull trap (it is a flash of negative energy in pnp) finger of death and horrid wilting as well, but most if not all of them are susceptible in bg.
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It would be great if those latter immunities were implemented in the game but I doubt that will ever happen.