So I'll go back to my guess that the application layer is the same but the
underlying layers (transport to physical) have been altered to NMEA 2000.

Incorrect. The bridge cable is passive. The NG cabling, it appears, just
has extra wiring to accomodate the older SeaTalk network. I'm guessing they
did this to allow bringing older SeaTalk gear into the network without
having to run a lot of additional cabling. That is, bring a line off an
older SeaTalk-based transducer and tee it into a new NG network. Leaves
them an opening to attaching existing gear (sensors, gauges and the like) to
the new network. So if you pulled a new NG 'backbone' network you would be
able to connect any older gear onto the cable without keeping the old
network.

It would not have made much economic sense to go with the NMEA2k protocol
stack and not take advantage of the economies of scale offered by
standards-based chipsets.

-Bill Kearney