Obtaining angular velocity from quaternion data (BNO055)
I do not know how to interpret the data given by that sensor, but from
reading the code, the meaning of dt is pretty clear.
The function you show assumes that the sensor started in the reference
orientation (quaternion one), then rotated at a constant angular
velocity until it reached the current orientation (quaternion *this),
and this rotation happened over a time dt.
Then dt is the time required for moving from the reference orientation
to the orientation described by the quaternion *this. You should
probably check out the sensor's datasheet for more details on how the
orientations are provided.