Assembly was never portable, or do you think 6502 on Apple would work out of the box in a Gameboy or C64?
In 1981, one could write a z80 assembly program for cp/m and it would run on thousands of different computer models.
The article is perfectly clear:
> The good news is that it is very easy to write assembly which targets Apple’s computers as well as the other 64-bit ARM devices running operating systems other than Darwin.
Would be tricky to have your 6502 code running on a Game Boy as it has a Z80/8080.