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To be fair, Diancie is a mutation of Carbink in-universe, just one that is unknown to humans, so the player can't trigger it. So your belief was actually not that far off.

I used to believe that Thunder Punch was the only Electric-type move that could damage Ground-types simply because of the punch aspect. Luckily, this belief of mine didn't last long because I actually tested out my theory as soon as I obtained an Electric-type that could learn Thunder Punch.