The comment belong to a previous version (not commited) where the rate_correction calculation was:
 ahrs_impl.rate_correction.p += (residual_imu.x / inv_rate_gain) << 5;
So the proportional gain "Kp" of the complementary filter was: Kp = 1/ inv_rate_gain / FRAC_conversion * 32

Now the comment needs to be corrected:
Kp = 1/ inv_rate_gain / FRAC_conversion
 inv_rate_gain = 1 / (2 * mag_zeta * mag_omega * AHRS_PROPAGATE_FREQUENCY) *
                AHRS_MAG_CORRECT_FREQUENCY / FRAC_conversion
inv_rate_gain = 32  * AHRS_MAG_CORRECT_FREQUENCY /
               (2 * mag_zeta * mag_omega * AHRS_PROPAGATE_FREQUENCY)
inv_rate_gain = 16 * AHRS_MAG_CORRECT_FREQUENCY /
                (mag_zeta * mag_omega * AHRS_PROPAGATE_FREQUENCY)