There were blocking divisions tasked with preventing desertion. They did not sit behind half-unarmed rifle divisions machine-gunning them if they retreated. Instead blocking divisions were made up of seasoned, reliable troops. Their job was to help stabilize the line. They'd stay either on or just behind the line. If they encountered soldiers who were lost, separated, or fleeing they would send them to assembly areas to be re-joined with their units. Some fleeing soldiers were executed for desertion, but that was a small percentage. If blocking troops intercepted 100 fleeing soldiers then 1 of them was executed, 10 were tried and sent to penal units, and 90 were returned to their units and sent back to the front line.

They served other purposes like protecting rear areas from infiltrators and helping with communications and logistics. And they were all fully equipped soldiers who often fought on the front lines themselves.

The notion of Soviet "Not one step backwards" orders is oversold. There was never an incident where Red Army soldiers were machine gunned by their own side while trying to retreat, and to the best of my knowledge there were no mass executions for retreating when the situation was untenable.

Also the "One man gets ammo, one man gets a rifle" thing never happened. The supply situation was never so dire that soldiers weren't given rifles and ammo.

Overall the movie undermines the heroism and valor of the people who fought and died defending Stalingrad, and it's really very crass. They were heroes and they deserved a better and more honest story than what Enemy at the Gates gave them.