Metal/Dark Energy

[Muku Aku]

I was under the impression that the offensive and defensive boosts gained by Dark and Metal energy, are only gained by Dark and Metal type PKMN only, respectively. However, I keep seeing strategies that include attaching one of these two energys to a PKMN not of the type. Is there something I'm misunderstanding?

[PokePop]

Depends on how old those strategies are.
Until the middle of last year, they kept their bonuses (with drawbacks) on all types of Pokemon.

Now, if people are still trying to use them by the old versions of the cards, they are mistaken.