Yes technically they are not fully preemptive in the sense that an os thread is (the os sends an interrupt which halts the processing at the CPU level), but in both go and elixir the programmer has no control over when the context switching happens, and "function calls" which are the yield boundaries happen all over the place, so it's "effectively preemptive".
Elixir is in practice more preemptive than go (last I checked with go) because you cannot infinitely loop and lock cpu in elixir -- a loop requires you to tail-call in elixir, so that's a yield boundary, and I'm certain that in earlier go that wasn't the case if you `while true {}`
