We're in the process of adding legacy moves to the Global Pokedex. Pardon our dust. Things will be clearer once we're done. :)
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Unless I missed smth they nowhere say it's a move he can learn, just have. On the flip side they nowhere make a distinction between legacy and current moves, which may confuse players, specially new ones.
E: Seems a mistake indeed cause no other legacy moves are listed on other pokemon.
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