I don't understand the bit about the AM radio. Surely he must have meant something in the range of 0.5-1.5 MHz, which is the typical band for AM. I don't think a radio broadcast receiver could pick up an EM signal in the sub-20 kHz range.
EDIT: Thinking about, maybe what he meant is not that the carrier signal is in the sub-20 kHz range, but rather that the switching happens at around that frequency. For example, gates opening and closing modulating the amplitude at audible frequencies.
The Altair had a 2Mhz clock.
By the mid-1990s, this was in the FM broadcast range, as a sysadmin I knew at the time demonstrated with an open-cased tower he was working on. (Perhaps perpetually.)