Hi Anton,
Thanks!
The short answer is "yes" the IMU is used for calculation of the position (lat, lon, alt) as well as controlling the autopilot. More specifically ArduPilot has an EKF that combines the IMU (accelerometers+gyro), GPS, barometer and compass to calculate a new position estimate at 50hz (or higher) and this estimate is used both for control but it also appears in the DPTH onboard log message that can be later uploaded to ReefMaster.