That was a common way to code in BASIC on the commodore 64. It's not 100% in BASIC, sure, but you can get it to list the program contents on the READY prompt. "8-bit show and tell" has a YouTube video on it.
I just posted the link to the 8-bit show and tell video, right before seeing your comment. I agree. The bulk of the logic of the game seems to be BASIC, with the bits that'd be too slow otherwise sped up with machine code.