it’s hard to tell what this has to do with plan 9… what is “go and plan 9 assembly”?
Plan 9 had its own assembler and the one in the Go toolchain is based on it, which might explain its use here. Like you I’m not sure why the prominent billing in the title of this post, the actual page doesn’t make a huge deal out of it.
IIRC, plan 9 assembly is platform independent; the plan 9 assembler takes it as input and outputs platform-specific machine code. This means it’s more portable, basically.