It's hard to get good clear info on their study. It does not look like they have published the results of their "Spry Belt Study". You can see the study design here: https://clinicaltrials.gov/study/NCT03986203?tab=table. It looks like they presented data from the study at a couple of conferences. "BHT presented data from their clinical trial at ASBMR 2023 and ENDO 2023".
In a blog post on their website they say this "The primary goal of the study was to provide quantifiable measurement in the change in vertebral strength using Biomechanical Computed Tomography and finite element analysis. The participants in the Active Treatment group who used the device a minimum of 3 times per week throughout the year experienced an average bone strength loss of 0.48%. In contrast, those in the Sham group lost 2.84% on average, indicating a relative difference of 2.36% (P=0.014). This represents an impressive 82% reduction in the rate of bone strength loss among the Active Treatment group."
So much less bone strength loss but not a gain. I couldn't find DXA score data.

As to osteopenia vs osteoporosis, I suspect this was a financial - marketing - FDA approval type of decision to shoot for the osteopenia market. Probably not a scientific one where there is some reason it works for osteopenia and not osteoporosis. That's just my guess. I'm not sure what to think of this device at this point.