What he is talking about is how fast the membrane/cone can start and stop moving.
Having a driver that goes faster than that is pointless if you can’t hear it.
Have a listen to the Pasma tweeters I have >5khz, it’s the mass they have which is zero as it’s gas, and the extension out to 100khz that gives them a sound that just floats in front of you like fireflies.
https://www.ebay.com.au/itm/Magnat-Plasma-MP-02-ion-tweeter-massless-air-plasma-speaker-Plasmahochtoner/272894087984?hash=item3f89c15330:g:V5sAAOSwCJxZ6PXy&frcectupt=true
Cheers George