> the latrines are always at least 30m from the sleeping quarters

The study did show positive health outcomes with the new housing over a traditional mud + thatch dwelling.

Given that the new housing incorporated dedicated latrines, harvesting of clean water, and insect-exclusion techniques, it's unsurprising that the new housing outperformed traditional dwellings in health outcomes.

It didn't do a cost-benefit analysis comparing an equivalent investment in e.g. provision of latrines, insect-netting, clean water, or simply providing cash to the participants.