The SAE test protocols take a lot of factors into account when tow rating the trucks. One of those factors is coolant temperature rise during the test run. This is why in 2020 GM put that giant 21" fan on their trucks, along with a bigger trans cooler, thus allowing them to increase the towing capacity.
https://fifthwheelst.com/SAE-J2807-Tow-Tests.html
https://www.motortrend.com/how-to/15...-the-standard/