And at the time Plan 9 happened it was hardly re-inventing anything either.
Intel's Asm syntax was defined in 1978 with the release of the 8086, and the 32-bit superset in 1985 with the 386. CP/M, DOS, and later Windows assemblers all used the official syntax.
For context, gcc was first released in 1987 i.e. about the same time that Plan 9 started.
Go authors didn't attempt to re-invent asm syntax. They re-used the work they did over 30 years ago.
And at the time Plan 9 happened it was hardly re-inventing anything either. It was still the time of invention.
References:
* https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs
* https://en.wikipedia.org/wiki/GNU_Compiler_Collection