Attacks can have two types of effects:
* Damage, like 10 HP, 20x HP, etc. including everything you do to calculate what that damage should be.
* Eveything else, including damage counters, poison, and miscleanious things like picking cards up out of the discard.
Attacks can have different targets for different parts of the attack:
* The defending pokemon ... that's your opponent's active.
* Your opponent's benched pokemon ... any or all of them.
* Your active pokemon ... the one that's attacking.
* Your benched pokemon.
* Your opponent himself.
* You.
Normally you do damage and then resolve all other effects of an attack. There are exceptions though. Blissey is one of them. Assuming that you play Blissey.
Happy Egg does the following:
1. Pulls an energy from the discard pile. This is an effect on Blissey by itself.
2. Calculates and does damage. This is an effect on the Defending pokemon.
However Holon WP specifically prevents all effects of your OPPONENT's attacks to the pokemon it's on. It does nothing about your own attacks effects on your pokemon (nor about opponents attacks that target YOU like Absol discarding a card from your hand).
So in short Holon WP does not prevent the effect of picking up the discarded energy.
