Cumulative damage isn't a bad rule, and it's actually pretty easy to understand. But this is a module with one pit trap that is 30 foot deep. It would be trivial to say "characters who fall in the pit take 6d6 damage". It could even just say "The pit is 30' deep. Use you normal rules for falling damage" since, after all, the module is not specific to a particular OSR rule set.
Instead, the text for the pit says "Pit Trap 30 feet deep, with the person falling into it taking 3D6 Damage from falling." Not only is the grammar terrible, but it completely misrepresents the damage, requiring the brain screw of a paragraph that I quoted above.
It's like there was a contest to make the simplest trap ever and describe it in the most convoluted way possible.