Evolution of the Nuclear Type

[Sparcely Dun]

Besides it wouldn't be logical that if the type become less potent, it becomes more powerful against a type it didn't affect before.

You're right about the gaps in logic, especially the Normal type thing (although you could argue 'normal' as the opposite of mutated from radiation? But that's a stretch) but Steel used to resist Dark up until gen 6 so it wouldn't be the first time.