Hello, the app uses the same method for evaluating movement data.
The results were compared to PSG (How we measure your dreams - Sleep as Android), and the algorithms for sleep phases estimations were trained on sets with SleepLab results.
Here are some of the studies evaluating sonar technoglogy for sleep monitoring:
https://www.researchgate.net/publication/334164490_Sleep_Staging_Monitoring_Based_on_Sonar_Smartphone_Technology
https://www.researchgate.net/publication/334164545_Detection_of_Sleep_Apnea_Using_Sonar_Smartphone_Technology
The crucial aspect for accurate sleep monitoring with sonar is good sonar signal and distance between you and the phone. If you feel your phone is giving you strange sonar data, try different position of the phone, and lower the distance. You can also try a different frequency in Settings > Sleep tracking > Test sensor (from the drop down menu list).