Yes, all old NES games were written in 6502 assembly (named after the NES's 6502 processor), and even most games into the Super Nintendo and Game Boy days were written using assembly language.

The source would've looked very similar to this, although I can assume the original labels would've been in Japanese. The difficulty in creating a disassembly like this isn't converting the machine code back into assembly, which can be done rather simply, but instead re-adding all the label names, which are lost when the game is built. It's quite the undertaking, and the author must know the complete game back to front.