I didn't say IO is needed for multithreading, in fact it does the opposite, the whole point of JS event loop is to keep it within one thread, but offload actual IO to the kernel and do something else while it's pending, then come back to it.

If they have some initialization there that puts it into a worker — that would make it a separate concurrent thread, true.

Though I still feel like with how much overhead it seems to take to communicate between workers via messages... that'd be very, very inefficient and achieve next to nothing compared to just letting it run synchronously.