I think Herr Willi M was more concerned with actually retaining pitch control and also the finite structural strength of the aircraft at mach numbers higher than .86 especially if the aircraft was steadily 'bunting' due to mach tuck etc !
The Hawk has the advantage of great structural strength and of course irreversible fully powered tailplane and ailerons... and a fairly good bang seat if it all goes T.U.
rgds LR
edit - also -depends how you interpret this phrase ?
the airflow around the aircraft reached the speed of sound