Let me see if I can understand something here. semi-trucks and trailers have air brakes. Air brakes mean there are spring brake chambers which require air to be released this is true the world over. Meaning there must also be air pressure in the tanks so just stepping on the brake pedal should stop the rig. before a huge amount of momentum was built up the unit was being towed by a single cable this means there had to be a driver to steer it. He had a minimum of 2 ways to stop the unit 3 if you could count putting it in gear Sometimes not an option turning and engine in reverse can cause not good things to happen.
The only way that truck trailer combination would not have any brakes would be if there was no air pressure and the spring brakes have been what is called caged open. in which case then you would need a towbar or at the very least a pipe with the cable run through it because if the towing vehicle stopped the towed vehicle would ram into the back of it. I saw no pipe SO why couldn't the driver stop it before it rolled backwards? I can think of only 1 reason they were not familiar with where the parking brake button or lever in the case of Asian and European trucks was located. On them instead on being in plain easy reach on the dash they are sometimes near the floor at the side of the seat not always in the best position to be activated without having to reach down.