A mining rack would mean the metal frame where the 4090's are mounted, not the motherboard. Since they pulled off this build, We will assume they are smart to use a server motherboard with enough PCIe slots and lanes.
Ahhh. I was thinking they might also have been meaning one of the mining specific motherboards, which commonly use a bunch of PCIe x16 physical slots with only PCIe x1 electrical connections.
