Not to discount what he did, but he didn't write the game in Assembly. He wrote it in C then decompiled it to Assembly and used that as a starting point for the Assembly code.
From the article:
>Starting in assembly right away would be a bit too insane, so I first wrote a reference implementation in C
As someone who started in Assembly right away creating several games with a much simpler 6510 CPU, I can vouch for the fact that starting in Assembly would be a bit too insane especially on modern CPUs.
> Then I began slowly porting everything to handwritten assembly.
How did he make the reference code? I'm reading that as he wrote an implementation in C, then decompiled it and then cleaned up the decompiled Assembly language code.
Maybe I'm wrong. How do you take his statement above?