Isn't BioShock built off one of the Unreal engines? That would also have more or less built-in LAN/dedicated server support. So in essence they'd have to strip that out which would make even more work. I realize that BS was made as a single player game and quite a bit of effort would be involved to change the game mechanics to support it but their excuse sounds pretty weak to me.
Not that it really matters to me as I'm only interested in the single-player aspect anyways. So I'm just hoping that didn't suffer at the hands of making it multiplayer.