They use IMU sensors not cameras. The linked article is a bit bad in that it does not make it very clear and it has a quite misleading hero image (in the tradition of useless hero images). They try to estimate pose in detail through IMU signals. While pose estimation through IMU is not new, I am not sure if something like getting into that deep detail (out of a general classification) has really been done.

For us normies, IMU = Inertial measurement unit

https://en.m.wikipedia.org/wiki/Inertial_measurement_unit