That is correct, the lockup clutch is only engaged in 3rd gear and above(truck in motion), and it stays engaged 100% of the time untill the trans shifts to 2nd.
here is the input end of the Allison,3000 series, minus the housing.
It shows the input flex plates and the input flange on the left, which is connected to and spins the TC housing(green). Those bumps on the housing are what the transmission input RPM sensor(lower right front on trans housing), senses as they spin by at engine RPM. The TC housing connects to a hollow output shaft, which drives the accessory/PTO drive gear(red) and passes into the hydraulic pump(blue), all driven at engine RPM. The TC turbine is inside the TC pump/housing as is the lockup clutch, and drives the turbine output shaft(yellow) hydraulically, or hard coupled by lockup, thru that hollow TC housing/shaft. It spins the first carrier, and the Turbine RPM sensor on top of the control module senses the mid section/turbine RPM of the spinning section shown to the right in this pic.
View attachment 911035