That's certainly an ... interesting choice.
https://plan9.io/sys/doc/comp.html
IIRC before Go was self compiling, it was compiled using 9c, and its architecture inherits from 9c.
EDIT: some nearby threads got the details better!
Back in those days I actually found that, with a few small tweaks, I could compile the Plan 9 operating system using Go's C suite. We didn't pursue it further but this was one of the options we looked into for cross-compiling Plan 9 from other operating systems.
>Go uses its own internal assembly language called Plan9.
Plan9 is the name of the OS. You wouldn't name a programming language "Linux", even if Linus created it and it was super related or not at all related.
The article simply misspoke by saying that the assembly language is "called plan9".
Considering there is no introduction at all to this beyond "I discovered it's called Plan9", I'm assuming the author really thinks this is a language widely named "Plan9".