As a non-mathematical type I understood Doppler distortion to be caused
when a high frequency was generated by a driver already in motion with a
low frequency.

The example was a woofer moving full excursion on a very low tone while
generating a higher tone. Let's do an extreme case of a 10 Hz excursion
and a 1000 Hz tone. Every 20th of a second (change in direction at 10 Hz)
the pitch of the 1000 Hz tone would change as its vibrating medium (the
woofer cone) changed from moving toward the listener to moving away.