Perhaps REST versus WS is really about private and public interfaces.
That’s exactly right, but exactly opposite of what he suggests; that SOA is better for public interfaces, while REST is better for private.
REST’s uniform interface is superior for public interfaces because its semantics are, well, public. Each new interface that is presented to a customer via SOAP/WSDL introduces a barrier to entry because the customer has to learn the semantics of that interface. Reusing a pervasively deployed one presents no such barrier.