From https://go.dev/doc/asm:

> The assembler is based on the input style of the Plan 9 assemblers, which is documented in detail elsewhere. If you plan to write assembly language, you should read that document although much of it is Plan 9-specific.

That first sentence links to https://9p.io/sys/doc/asm.html. So yes, definitely not a coincidence.

Go is the evolution of Limbo from Inferno, which was designed based on the failure of Alef on Plan 9, combined with a minor set of Oberon-2 influences.

Go itself is from the same people who worked on Plan9 (which is why I find the choice of name in the OP particularly confusing).

Yes. Two of Go's creators, Rob Pike and Ken Thompson, were also authors of Plan 9 at Bell Labs.

Yes, some of the same people created Go.