Yes, that sounds right, Bob. An adapter cable built with coax would provide much worse noise rejection than one built with shielded twisted pair, because of both the lack of physical symmetry between the two signal conductors, and because of the need to tie pins 1 and 3 together, to provide a connection between the chassis of the two components through the shield. That in turn would amount to a similar situation as a single-ended interface, in which inter-chassis ground-loop currents flow through a common path with signal return currents.
Which raises a good question about how typical adapters are designed internally. I'm not sure what the answer is.
Best,
-- Al
