Surely everyone can hear tones below 20Hz? The buzz of machines like refrigerators? The hum of traffic? Airplanes, etc? Most urban areas are saturated with these irritating frequencies, and I hear them all day long. Am I really so unusual?
If I play 20Hz in Audacity, it's very loud, clear, and easily perceived. It fades in strength as I go down through the teens, down to 9Hz, which is feint, but I can definitely still hear it. I wasn't able to hear 8Hz, but I was listening in a noisy environment and haven't done much testing. Plus, my ears are pretty blocked up with wax and I have tinnitus, so I wouldn't say my hearing was good anyway.
At the upper end, I can't hear much above 10Hz. So to me, the claim that humans can hear 20Hz–20kHz seems perfectly correct if you halve it!