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.
For another interesting read there's the guy that disassembled Robotron, and traced the code out by hand across 512 printed pages of assembly and fixed 2 long-standing bugs.
I imagine the original labels would have been in English. The assembly source code I've seen for Japanese games has variables and labels in English with Japanese comments in Shift-JIS. I would guess the choice was forced because the assembler, linker, debugger or other tools did not support Shift-JIS properly. Often labels are restricted to 6 bytes which would be 3 Japanese characters. Perhaps such a limit was also a factor.
The first console game I can think of that was written in C was Sonic Spinball. Compared to other Sonic games, the jank was palpable. But the dev team was way behind schedule and behind the 8 ball; switching to C helped them crank out the engine much faster.
Yes I believe on the Genesis most games were written in assembly as well. Sonic Spinball was written in a “high level language” (i.e. C) and so it only runs at 30 FPS instead of 60.
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.