You are reading that correctly, however, it is likely a limitation of the technology they used in the study. Strangely, the didn't mention how they decided on

You are reading that correctly, however, it is likely a limitation of the technology they used in the study. Strangely, the didn't mention how they decided on stimulation volume, however, most studies will either set a fixed volume for the study, or measure the users hearing while they are awake, and then set a fixed volume for that person. Our technology (and we're not the first to do this) adapts the volume based on brain response during sleep in real-time. When you don't do this you risk either having the volume so low that it doesn't evoke a response, or so high that you decrease sleep depth, and don't get the correct response. Therefore, anyone who did not get the appropriate volume would end up as a non-responder. It is also more challenging for previously used algorithms to detect a slow-wave in older adults because the delta power is lower, so some of these participants may have had limited stimulation opportunities. We've developed methods which improve on the state of the art, but we have not validated those in a study yet.