Professionals that see "rackmount" and don't stop at a one-unit rack. Industry-scale customers, not prosumer.

I don't think he intends to make a DGX Pod out of Macs, but the Studio is barely scalable. If we still had Mac Pro PCIe, then at least you could use that for interconnect. Thunderbolt RDMA is such a low pittance that Apple is basically only competing against novelty hardware like the DGX Spark.