Did they actually redo everything, or does it just look that way from starting from the Plan9 toolchain? Which could also be said to be re-doing everything, but from a much earlier starting point.

IIRC Go started out shipping with a port of the Plan9 C compiler and toolchain - it was bootstrapped by building the C compiler with your system C compiler, then building the Go compiler. Which, until re-written in Go circa-2013, was in Plan-9 style C. It all looks deeply idiosyncratic but it was a toolchain the initial implementors were highly familiar with.