No, in this case he is quite correct.
As we have been saying, the aircraft flies relative to the air around it. If the air suddenly changes velocity relative to the aircraft (note the ground doesn't come into it!!) you have winshear.
The aircraft does have momentum which is mass times velocity. Velocity is relative to the observer. The velocity that counts in this instance is that relative to the airflow. A larger aircraft has more mass, therefore more momentum.
Ground speed is a useful reference in this case, as the magnitude of the change in the airmasses velocity relative to the aircraft will be the same as it's change realative to the ground.
You could, however, reference the aircrafts speed relative to the moon, sun,or alpha centaurus with the same result, it's just that the ground is nearer and therefore more convinient!!