Ah, now I see these things changed recently. I wrote original comment because at the time of Go 1.1 or 1.2 my colleague wrote racy code which accessed global variable without mutexes. I tried to educate him to use mutexes, by reducing his code to the minimal version demonstrating race condition, but failed. Then I digged into the source code of Go runtime library and discovered that only one thread could be in the running state at any moment (other threads could be in the state of blocking system call).
So, things changed since that time. OK, nice to see Go evolves. Interesting, how many code in production got broken during upgrade to Go 1.5 due to this backwards-incompatible change. Overall situation seemed totally reasonable to me at that time: Go is a language for average programmer, so it should prefer safely over runtime performance; and coding cowboys who are comfortable with mutexes, interlocked instructions, memory barrier instructions, and lock-free data structures will use C/C++ anyway.
But that wasn't true either at the time of Go 1.1, nor ever I believe. It was the _default_ behavior to have a single core running at a time, but you could just tweak that with the [GOMAXPROCS](https://godoc.org/runtime#GOMAXPROCS) variable, documented everywhere. (That's what changed in Go 1.5; now the default for GOMAXPROCS is the number of cores in the machine.)
Your statement holds only if GOMAXPROCS is 1, which granted used to be the default, but was always able to be increased, and the default now is the number of cores on the machine.