Those are mostly a bazillion nodes with a couple dozen Xeon cores each, wired up together (over a very nice interconnect). Each Linux instance only has to handle the cores for a single node.
That said, I agree with you that it isn't particularly surprising that Linux handles lots of cores fine, it's been run on more esoteric hardware for ages.
That said, I agree with you that it isn't particularly surprising that Linux handles lots of cores fine, it's been run on more esoteric hardware for ages.