This was a paper from 2013 though, 7 years ago. I'm sure they've more than made it a proper technology now, with all the compute power and deep learning and what not. Maybe unrelated, but WhatsApp does monitor your phone accelerometer data 100% of the time, even when it's in the background. An app doesn't even need to ask for permission to get access to the accelerometer data, so there's not even a pop up of any sort.