You're assuming that when they say "912 computers per rack" they mean 912 4-core servers per rack. It's a reasonable assumption, that's how I interpreted that statement when I read it the first time. Nonetheless, I'm quite sure that that interpretation is incorrect.
If you watch the video carefully, you'll notice that their rack/case contains 16 cartridges, and each cartridge has 18 servers on it. That indicates they have 288 servers per rack. 288x4=912, so I believe that when they said 912 computers per rack, they actually meant 912 cores per rack.
If you watch the video carefully, you'll notice that their rack/case contains 16 cartridges, and each cartridge has 18 servers on it. That indicates they have 288 servers per rack. 288x4=912, so I believe that when they said 912 computers per rack, they actually meant 912 cores per rack.