They can TEST gear that can. Try thinking 3-Dimensionally. If I need to test a piece of gear that supposedly is flat up to 20kHz....what the hell am I gonna test with? The further below 20Hz and above 20Khz a device can reproduce flat responses, the flatter IN the audible range it will work. Theoretically.
But you are correct in the hearing range but it is actually more like 16kHz.

I also think BMXRACER was being a bit facetious....you don't honestly think he was serious? I thought it was funny actually cause he can't be....try on the humor hat once in a while...a whole new world opens up.