Comment by emptybits
I agree; I read the headline as meaning a 6502 assembler was written, as opposed to 6502 assembly being written.
Writing compilers gets difficult, quickly. But writing assemblers is/was common enough for simple architectures and it's quite fun and relatively easy to prove/test for correctness. At least compared to most compilers.
One of my first gigs out of school involved writing a Z80 assembler because I "needed" nonstandard (Sharp/DMG/8080) instructions to be handled during a codebase port. It was enjoyable! I recommend everyone write an 8-bit assembler at least once!
Still, TFA is very interesting and I appreciate OP's share. :-)
I wrote my own 6809 assembler [1] largely just because, and I ended up adding a 6809 emulator to it, so I can run tests during the assembly phase. I'm only aware of one other assembler (a 6502 one) that can do this. It's a fun project.
[1] https://github.com/spc476/a09