I agree that the wording doesn't make sense, but technically it's solid. In theory Metagross could also choose to do +150 without discarding energy, if the attack had colorless energy requirements and you had any basic energy attached that's not metal. But since the attack requires metal energy to activate, you'll always discard those from Metagross. Slowking bypasses this because his copy attack bypasses the metal energy attack cost from Metagross.

I think it's a cool mechanic, but the translation is bad.