Actually the statement "Hubbard was a geek first and a musician second" is facetious and untrue. In fact, he worked as a professional musician before he worked on computers.
As for writing music in monitors, at the start of the 64 era, this wasn't only done, it was normal if you wanted to source your own tunes. In fact, programmers often began programming in assembler and writing music before they even had a monitor, they'd Poke every byte into RAM from BASIC, then re-write the basic program so it was just a system-call to run the program, then save the lot to disk.
Some simple early assemblers were written this way, those assemblers were used to write better assemblers and these second generation assemblers were used to write games and graphics and music programs: From the moment you switch on a shop-bought C64 you have enough there with BASIC to bootstrap any project you like. That's one of the things that was great about it: A proper self-start home-brew system on which the cottage industry which began the games industry was based.