I do understand, was just mobile and in a hurry so I failed to fully qualify the statement.
What I was saying is if you're in the WASM toolchain/ecosystem, something like pthreads would already be implemented by Emscripten[0]. In other words, spinning up web workers is handled for you seamlessly. It's all dealt with at a very low level of abstraction. You're literally using the SharedArrayBuffer as memory.
Contrast that to JS, and you have to serialize/deserialize types before they can even be stored in SharedArrayBuffer. It's costly and far from seamless.
As an aside, it's unfortunate SharedArrayBuffer was hit so hard by Spectre. The poor standard is cursed or something.