Let's not forget their attempt at inventing yet another Asm syntax for x86, when there is already the horrible GNU/AT&T as well as the official syntax of the CPU documentation.
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.