Just a guess, but since they didn't make very many of them and only sold them new to fleet customers for "upfitting." Gm probably didn't bother going through the j2807 towing tests to determine an actual gcwr for them. They probably just used the regular (1500) suburban gcwr hence the high payload but really low towing capacity.
Bingo - that makes total sense.
I did some searching for GM literature to see what the GCWR is for the "standard" 2017 Suburban - and found the Chevrolet 2017 Towing and Trailering guide:
In section 13, Colorado , Tahoe , Suburban , etc - it says that the Tahoe/Suburban with the EcoTec 5.3: V8 with Max Trailering Package - has a GCWR of 14,000 pounds.
So with the GVWR of the Suburban 3500 being 11,000 pounds - that gives you the 3000 pound tow rating.