Actiblizz do not write their own netcode. They get a bunch of Irishmen to do it for them.
>Primary products developed by Demonware include the "Demonware State Engine" and "Matchmaking+". The State Engine is a high-performance state synchronization C++ programming framework that eliminates the need to reinvent netcode in multiplayer games. Matchmaking+ provides services for multiplayer games such as matchmaking, user profiling, and gaming statistics. Demonware's main product, which is used for multiplayer in the Call of Duty franchise (among other games) is programmed in Erlang[3] and Python[4]