Carmack is famous externally, especially among a generation that grew up with Doom/Quake. But it isn't like Facebook employs a bunch of ordinary engineers and then Carmack. There are a lot of people with extraordinarily impressive resumes and a long history of massive technical success there. It is not a surprise to me that it didn't just become the "do what Carmack says" show when he joined. Consensus-building becomes even harder at larger institutions.