`fork` is how you create a new process on Linux. Threads, which GP was talking about, are different, especially Go threads, which are not the same as OS threads.
I believe GP was referring to the `clone` system call, which both fork and pthread_create use under the covers. The difference is what happens to the memory-resident data- the forked/child program shares the address space of the parent until either writes to a page, in which case the page is copied such that each process gets its own unique copy. This is known as copy-on-write (COW). Pthreads, on the other hand, outright share the process's address space and must implement their own synchronization via locks or whatever.
Obviously, the performance of fork vs. pthread_create could differ dramatically depending on what the program does.
Goroutines are a layer of abstraction above this. They might run on different threads concurrently- the Go runtime controls what happens here and may differ on various architectures / OSes. If you break into a running Go program on Linux with gdb, there's definitely a bunch of pthreads running, maybe for goroutines, and probably for garbage collection, and other stuff. (If you want to actually debug go code, you should of course use something like delve).
