There definitely was used windows also you can see dummy plug on every card as a proof of windows. So nothing more here to tell, people runs fine even 8 GPU on one motherboard so you can have there 4xHD6990 or 4xHD6970 no difference.
Seems I don't understood something here or we talking about some nonsense. I meant that on that config they have 1 motherboard and few card config. 1 motherboard will have 1 os. As Rizwan noted they also using Ubuntu, maybe because were testing linux client or just wanted to be legal