Yaw fallback in T265 based navifation

I have set all my EK3_SRC1 params based on the output of visual system (6). and all the EK3_SRC2 params to GPS and EK3_SRC2_Yaw as Compass.

before switching from SRC1 to SRC2, I want to sen the “VISO ALIGN”, which calls the function

bool AP_VisualOdom_IntelT265::align_sensor_to_vehicle(const Vector3f &position, const Quaternion &attitude)

but this function has a check

    if (AP::ahrs().is_ext_nav_used_for_yaw()) {
        return false;

so even if I call this function, it will exit since I am using t265 for yaw in SRC1. I am curious, why is this check needed?
and based on what params the AHRS yaw is calculated?

   _yaw_trim = wrap_2PI(AP::ahrs().get_yaw() - sens_yaw);

@rmackay9 ,your help is much appreciated
references - VIO alignment in primary lane - #9 by sbaccam
Intel RealSense T265 — Copter documentation
External Visual Odometry and GPS at the same time? - #7 by rmackay9