In order to make the game small enough to fit on a cassette tape they had to ditch basic and program the entire game, world in assembly.

Putting aside the fact that the majority of commercial games of the time were written in assembly (or other low-level languages) just as a matter of course, I strongly suspect that programming the game in assembly was an execution speed issue, and not a cassette space issue. Regular audio cassettes easily held enough data to fill an average 8-bit home computer's memory many times over, whether that data was machine code or BASIC instruction codes.