I should have pointed out the errata, however it looks to me like the example being given has Mesprit LvX attacking a Pokemon that has an Unown G attached to it. The errata was issued to clarify that a Pokemon with Unown G attached would not be protected from an effect of its own attack done to itself, like having to discard energy to do the attack. In the example, if Mesprit LvX is not the Pokemon with Unown G attached, even without the errata, the wording on Unown G makes it clear that it only prevents effects done to the Pokemon it is attached to. If it is not attached to Mesprit LvX in this case, there is no way the Unown G would apply to the Mesprit LvX energy discard. Reading the card is always the best place to start when trying to figure out a card effect.
BDS