From PokéBeach's Eidilon Forest spoiliers:
If a player discards his/her Pokémon, would those discarded Pokémon be considered Knocked-Out?
Giant Stump - Trainer - Stadium
Each player can only have 3 Benched Pokemon. If you or your opponent have 4 or more Pokemon on your Bench when this card is played, discard Pokemon and all cards attached to them until there are 3 left.
If a player discards his/her Pokémon, would those discarded Pokémon be considered Knocked-Out?