Garmin NMEA 2000, not really!
The damn devil is in the damn details. Imagine my disappointment when I hooked a whole network of Maretron sensors into the Garmin 4212---no patch cable needed, much plug'n'play goodness expected---and got NOTHING! I can still scarcely believe it, but when I checked the installation manual, I learned that Garmin is hardly supporting NMEA 2000 at all. The page above says, "The GPSMAP 4000/5000 series unit...can receive data from a NMEA 2000 network...to show engine specific information...Also, the unit can receive heading data..." That seems to be it; no other standard data understood, and no data going out. Garmin even screwed up plug'n'play, for some reason needing "Device Instance" and "System Instance" specified, which I've never seen before.
In short---aside from using the right connector, and the right name---Garmin is not even close to supporting NMEA 2000 in the way we've come to expect from Simrad, Raymarine, Lowrance, and others. I am so disappointed.
I'm planning two major electronics upgrades at the moment. I have and have used lots of Garmin equipment and like it a lot but had not considered their earlier systems because of lack of NMEA 2000 compliance and compatability. I was strongly considering the 4000 series, on the understanding (similar to yours) that they had finally decided NMEA 2000 compatability/interoperability was the way to go.
I hope what you've found does not indicate Garmin's intent to maintain a substantially closed network infrastructure but, rather, is just waiting for a firmware upgrade to achieve full compatability. That's my hope...but at this point I'm not optimistic.
These are smart people with a successful business. But failure to support standards and allow the user to select peripheral equipment to be supported by central MFDs is very short-sighted.