Well I didn't find exactly what you're looking for. The only thing the service manual talks about flushing is the external cooler, not the transmission. Its in a diagnostic section for checking the cooler and lines to make sure it's all clear and flowing. The truck is not running, and the only thing the flushing system is hooked up to is the cooler and lines, not the transmission. If I'm reading it correctly.
The service manual says that if you want to change the fluid, drain the pan and refill. So, while nowhere does it say, "don't do a transmission flush", the manual says to drain and refill the pan.
Not sure if any of that helps, or the scenario you're in, but I read enough on this forum that flushing the trans is risky. Sure, you don't get every ounce of old fluid out, but a double drain and fill, while time consuming, is safer.
And, for whatever it's worth, I'm looking at the '17-'19 manual.
I can only guess that a flushing setup might be slightly faster in overall service time. Since the machine puts in the exact amount it takes out. When you drain the pan, you're basically somewhere from 6 quarts to 7 quarts. So you have to guess, refill, then go drive the truck and get it hot enough to do a "hot check" and topoff. Once again, a lot of guessing there, I'm sure some dealer techs on here can elaborate more.
Also, read the post #30 in the thread below. @ChevyTech77
is a verified dealer tech and helpful dude:
Should I plan on swapping out 4 or 5 gallons? Trying to decide on how much to order.