GM recommends 10%-15% tongue weight for conventional bumper towing. With the Silverado EV being so heavy they are probably basing the towing capacity on 10% tongue weight. So that would be 1000 lbs tongue weight and enough left over for the driver and about 100 lbs of payload.
Only the fully loaded RST will have a 1300 lbs payload. Lower trims with less weight adding options (no air suspension, no rear steer etc) will have higher payload capacity.
For the 20k lbs towing version they will just use a more heavy duty suspension like with their HD pickups for a higher payload capacity.
Side Note: The WT's lower 8000 lbs towing and 1200 lbs payload is most likely to keep the GVWR under 10k lbs for commercial regulation reasons.
Only the fully loaded RST will have a 1300 lbs payload. Lower trims with less weight adding options (no air suspension, no rear steer etc) will have higher payload capacity.
For the 20k lbs towing version they will just use a more heavy duty suspension like with their HD pickups for a higher payload capacity.
Side Note: The WT's lower 8000 lbs towing and 1200 lbs payload is most likely to keep the GVWR under 10k lbs for commercial regulation reasons.