It’s using the great war as an allegory for the supermarket war.
Just as the British and German empires went to war over Europe, Sainsbury and Tesco are at war over the high streets and out of town shopping centres.
While British and German civilians got conscripted in to one army or the other, purely by accident of birth, and got sent to fight each other, despite the fact that they had more in common with the men in the trenches facing them than they did with the officers behind them, so the staff of the major supermarkets are recruited by whichever one happens to have a vacancy at the time and are expected to show loyalty to their employer, when they would be better off joining forces with their rival’s employees and rising up against their capitalist overlords.