Right around 37 minutes in, Brevik mentions that Mike O'Brien (along with Pat, one of the three co-founders of ArenaNet) was the brains behind Battle.net, and then "a few of the guys from [Blizzard South] moved up north during the last six months of development and started making Diablo into multiplayer and integrating Battle.net into the entire thing."
That lines up with Pat's assessment:
> Initially Collin Murray, a programmer on StarCraft, and I flew to Redwood City to help, while other developers at Blizzard “HQ” in Irvine California worked on network “providers” for battle.net, modem and LAN games as well as the user-interface screens (known as “glue screens” at Blizzard) that performed character creation, game joining, and other meta-game functions.
That lines up with Pat's assessment:
> Initially Collin Murray, a programmer on StarCraft, and I flew to Redwood City to help, while other developers at Blizzard “HQ” in Irvine California worked on network “providers” for battle.net, modem and LAN games as well as the user-interface screens (known as “glue screens” at Blizzard) that performed character creation, game joining, and other meta-game functions.