If something will happen, then it happens. If WPM thinks it's the best decision to close the forums, then people should stop complaining about it and just let him do what he wants. It's his site after all. Sure it would be disappointing, but I'm sure WPM has a plan to make it up to us (like someone said, he could be building another forum).
Not only will your Sunkern NOT damage the Defending Pokémon, but the only effects of the attack will be self and own-bench damaging.
Slick's Sunkern 30HP
Flip a coin. If heads, Slick's Sunkern does 50 damage to itself. If tails, this attack does 20 damage to each of your Benched Pokémon. (Don’t apply Weakness and Resistance for Benched Pokémon.)