They had server infrastructure for an estimated 50,000 players. 18,000 players bought the game and were trying to play. Over 5x that number in pirates logged in, and because they didn't use any multiplayer authentication (accounts or some such), they had to support them like paying customers. In other words, going DRM free caused an overload on their server. It sounds like an underestimation of the amount of pirates they would get more than anything - they had over 250% capacity for paying customers.
Also, only Ubisoft claims piracy rates over 90% as far as I can tell.
Also, only Ubisoft claims piracy rates over 90% as far as I can tell.