The people in the backwards universe KNEW IT WAS BACKWARDS!!!!!
That’s why language used in the newspaper was things like ‘completed his sentence 2 years ago’ and the compère said ‘see you last night’ and they were fired before they had been performing for weeks. The guy was angry with Lister and Cat for eating his pie despite it being say there untouched.
Their brains worked forwards like the Boys from the Dwarf, they just couldn’t act in such a way which is why they found the eating forwards so remarkable.