After watching the video, I'd have to disagree- the discord didn't show up until after a sizable store of darkness quanta was built up, and two of them got stolen immediately. Thus, the discords didn't have that much of an influence. It might have swung the match (notice the "might") if a discord had shown up earlier. I'm not saying discord is OP- in a similar situation for, say, air, an early owl's eye could have swung the match as well. An important thing to remember about discord is that a larger quanta pool will better resist it than a smaller one- like at the beginning of a match. (plus, it creates a dangerous situation if the opponent has a pulvy, trident, or a creature with off-element abilities)
To be honest, I don't really care whether or not different decks per match are made mandatory or not- I just want to have fun ^_^