I guess he just used what he had on hand. By the time the freeze cartridges were out, you could get pretty good native assemblers (like Ocean's Laser Genius, which can even be scripted in a sort of Forth dialect). Then you'd use the monitor on the freeze cartridge to debug the code generated by the assembler.

I wrote many intros using the Power Cartridge monitor, before I learned about assemblers. For small projects it's fine!