I get how difficult a study like this is to carry out, but each participant was involved for just seven days, each night exposed to different conditions. The control environment was silence, and every other conditions did worse than controlled silence. In others they piped in fake environmental noises, pink noise, or made the participants wear earplugs with some other combination.

Eh. People condition to an environment, and someone conditioned to something like pink noise wouldn't have the acclimation issue (and they either specifically selected for people who don't use noise machines, or they just randomly got only people who don't), and it might drown out smaller environmental noises that otherwise would have disrupted their sleep. It would take a much longer study to determine this.

Or hey, maybe those insecure sleep masks tracking EEG and other things will give us some insights eventually. People just need to harvest the data from the other services.