Working from the 10K end-host number in the paper with a radix of 43 and 722 total router nodes, it’s the case that a given switch connects ~15 end-hosts. Fifteen downlinks to 43 uplinks seems pretty wasteful in its own right.

The numbers you cite are closer to realistic: a modified Clos fabric consisting of a four-way spine of 8-slot chassis with 36 100G blades connecting a full complement of 288 leaves, each with 4x100G up and 40x10G down (no oversubscription, at least nominally) leaves us with 11.5K hosts connected by a total of 292 switches. Even if the modular switches are 10X the cost of the ToR this is still markedly cheaper than 700+ ToR’s (and this doesn’t include actual power/cooling costs and the opportunity cost of space lost).

This also doesn’t include cabling/transceiver cost differences: 722 * 43 * 2 (62K) vs 288 * 4 * 2 (2.3K). That’s literally an order of magnitude difference.

Even if the design were approached using single-speed connections (ex: 48x10G up divided over 16 spines, 48x10G down locally) there’s still a pretty compelling numerical advantage to Clos: ~210 96x10G ToR’s and 16 16-slot chassis for 10K hosts is 226 devices and ~20K transceivers. If we assume the spines cost 10X the leaves then 370X is still almost half of 722X.