I'm not sure what the author meant by this. Python does support parallelism via multiprocessing (and in experimental versions, via threads in no-GIL builds), e.g. like this. It's a bit questionable whether it'd help here, because the overhead of sending the inputs between workers may be comparable to the speedup, but it's certainly possible, and very common in real tasks.

(I'm not familiar with Ruby, but from some googling it seems the situation is about the same as Python, but there's not a stdlib implementation and instead you need to use something third-party like the parallel gem.)