Stereotypically those were FDM so you'd send precisely two cycles of 1200 Hz or one cycle of 600 Hz and then a simple zero crossing detector would sync up and read the bits out.

I figured thats where he was going and was kind of surprised to see the multi-tone setup.

The more advanced ham radio guys are experts at using vaguely communications width bandwidths at often very lot signal to noise ratios to send as much error corrected data as possible.

20 years ago you'd demo something like PSK-31 using two COTS laptops just singing out the speakers and it would work pretty well. Newer modulation schemes are obviously faster / lower SNR / better error recovery / better PAR (peak average ratio) etc. A smart telecom engineer can run right up against the Shannon limit and flexibly trade off against limitations, either SNR or audio bandwidth or transfer speed or synch / preramble time...