Zoe is not really true dmg in the way it works for others. It uses post mitigation dmg as the number for the dmg. So you can still reduce that dmg with MR (MR - 30%). Just so that it won’t get reduced twice it uses true dmg.

They could have used pre mitigation dmg also, just like they do with Zeds R.

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Amumu is different to Zoe as he uses pre mitigation magic dmg and enhances it, so MR doesn’t do anything against his true dmg.