That's the app they used:

The phyphox app recorded acceleration and angular velocity at a sampling rate of 100 Hz.

I wonder if they considered using the fact that a roller coaster trajectory is closed loop? That tends to help with calibration, but maybe the devices are nicely factory calibrated to begin with. They get close, but stop short of mentioning it:

The measurements were stopped after the attraction returned to its resting state. This provides reference points at the beginning and the end of the ride where the velocity is zero.

And this gets you real close to building visual inertial oximetry:

Smartphone video recordings were used to correlate specific ride manoeuvres with the recorded data for the roller coaster.