Yes you’re right and that would be the case if both NDX2 and NDS were connected by the analogue outputs to the 252. As you’re using the NDX2 as a digital source to the NDS via the S/PDIF interface, this connection is transformer coupled (galvanically isolated). The setting of the NDX2 ground switch can be left at default (chassis). The NDS is providing the mains earth to signal ground connection in the system so it’s the setting of the ground switch on the NDS which needs to be set correctly.