Your 'application' that uses them can absolutely have data races (you also have to fight the GC on every interpreter not just one). I think what they mean is th

Your 'application' that uses them can absolutely have data races (you also have to fight the GC on every interpreter not just one). I think what they mean is that a local object within an interpreter without a shared (replicated) state does not have data races. Once you start message passing of course this is no longer true. Which isn't really different than a similar pattern of isolation in threading or multiprocessing.