I'm actually not a big fan of people who recite "concurrency is not the same as parallelism" like a mantra because I don't think it's anywhere near as orthogonal as those people think. But then, that's also largely because multicore is the norm now, rather than some bizarre exception. In a single core case, it is still true. Goroutines are just a different way of achieving async functionality, in a way probably a lot more convenient than the actual code of the time had, albeit at a bit of a performance penalty.

There was a period of time towards the beginning of Go when you could get some small performance advantages for certain tasks by locking the runtime to one goroutine at a time. They've long since addressed that, but there was a time when there were people writing Go code and deliberately limiting it to one execution context at a time.