> [..] it would be necessary to record it onto tape, such as the case presented here, which would have been fairly expensive at the time, and then, yes, use a tape loader accessory for a C64 at the time...the budget, it seems, for the average listener to have the gear to decode this, much less know to do it, seems to indicate an astronomically improbable level of discoverability.

Oh, no at all, distribution of software as audio was not exactly common but wasn't unusual either. In the Netherlands there was a radio programme that broadcast C64 programs you could record on cassette. Some printed computer magazines included software on flexi discs which were thin, flexible vinyl sheet records. Record players and double cassette decks, like the one in the the video, were ubiquitous back than. Double cassette decks were the means of choice for copying cassette software because this was much faster than on the computer. The Datasette was much cheaper than a 1541 floppy drive. Recording the audio from a record to tape and loading it via Datasette was a minor inconvenience compared to other methods of software distribution used back then.

The usual way to get new software, and with that I mean software you couldn't copy from your friends, was to just type it from a computer magazine. A considerable part of the old magazines was printed program code you could type in and run. I deliberately didn't write source code - it was raw hex listings. The typical form, if I remember correctly, was two columns with 17 hex pairs in each line. 16 bytes were program, one byte was a checksum. You would enter it in a program that would make an annoying buzzer sound when you hit enter at the end of the line and the checksum didn't match. The program I used was from a German computer magazine and aptly named checksummer, which is a nice wordplay on checksum and summer which is German for buzzer. So software on records was neither inconvenient nor expensive.

EDIT: Dug up an example:

https://www.retropoly.de/c64er/1988/64er_1988-05.pdf

One hex listing starts at page 42. Format is a little different from what I remembered, 8 bytes code plus 1 byte checksum, four columns of that on a magazine page.