> despite referencing the Intel doc.

I don't know why you would think referencing Intel's x86 software developer manual has anything to do with what syntax an assembler would use. Anyone writing an assembler, regardless of its input syntax would refer to it.

> ...but it's written in C and uses a parser generator? IMHO it feels a bit backwards --- and perhaps even a bit cheating if you're doing this for a "bootstrap pilgrimage"

Who says they share your goals? It says they wrote this just for "fun and learning," i.e. to learn how an assembler works and how x86 instructions are encoded, maybe even to practice their C.