The point is, to prove a point a DDoS attack can be initiated against Ubi's authentication servers thus making the game unplayable for extended periods of time for legitimate customers only.
Even if Ubi takes DDoS into account, the DDoS might slow the servers down just enough to keep kicking people out of their games.
Botnets aren't particularly expensive to rent any more; you'd have to be pretty committed to taking ubisoft down but I reckon it's doable.