Curiosity had me searching for information on this. I found this quote. To me it sounds like they just put no priority on Tow Capacity but were focused on Payload capacity, 4400 pound payload. You could park most cars inside and drive off. HA. The article gives the history. Based on the HD Frame, Brakes, Cooling, axles, tires etc. I would be very comfortable towing up to 10,000 with that vehicle regardless of the screwy rating from GM, just don't add 4,000 pounds of armor and then try to tow.
"That said, GM also offered a heavy-duty version of the Suburban, marketed as the Suburban 3500HD, starting with the 2016 model year.
Given the Suburban’s pickup roots, one would expect the Suburban HD model to offer the same powertrain and extra towing capacity of its Silverado HD counterpart, but this is only somewhat the case. The eleventh-generation Suburban HD began sales for the 2016 model year and was offered through 2020. It came with a 6.0-liter gas V8 engine making 360 horsepower and 380 lb-ft of torque; the same gas engine offered in GM’s line of heavy-duty trucks. No diesel engine was offered. While you’d expect it to offer the same towing capacity as GM’s full-size heavy-duty trucks, this isn’t the case. Rather, the emphasis is placed on increasing the vehicle’s GVWR and payload capacity. This is achieved through things like an upgraded frame, bigger brakes, heavier-duty cooling systems, and 8-lug wheels. In the Suburban 3500HD, this rolls up to a GVWR of 11,000lbs and payload capacity of a 4,405lbs; almost three times that of a regular 2016 ‘Burban. Towing capacity is greatly reduced though, down from 8,000lbs to 3,000lbs. The idea here is that the vehicle comes from the factory ready for upfitting by third parties. Many are armored and bulletproofed for use in tactical settings, though there are certainly other applications for a heavy-duty Chevrolet Suburban as well.
Unfortunately, the 2016-2020 Suburban HD was only offered to fleet customers. While it couldn’t be bought new by a private citizen, it is possible to find a 2016-2020 Suburban HD on the used market, though they’re tough to come by, and generally carry a high pricetag. As I’m writing this, there are just three Suburban HDs
listed for sale on Autotrader, with the cheapest being a 2016 model with just under 93,000 miles on the clock and an asking price of $44,685. That’s not cheap, but as GM has stated that it doesn’t plan to offer the all-new 2021 Suburban in an HD guise, expect these unique Suburbans to hold their values quite well going forward."