I'm not sure if Dasnake has a 6.7 Cummins or 5.9 but I know from reading my manual for my 6.7 that it states "If ambient temperatures are low and the coolant temperature is below 180°F (82°C), the engine idle speed will slowly increase to 1,000 RPM after two minutes of idle, if the following conditions are met:
• Foot is off brake pedal and throttle pedal.
• Automatic transmission is in PARK.
• Vehicle speed is 0 mph (0 km/h).
• Applying the throttle will cancel fast idle.
• Operating the exhaust brake at idle will greatly improve warm up rate and will help keep the engine close to operating temperature during extended idle."
My point being that idling with the exhaust brake activated DOES help with warm-up.