Yes, I should have been more explicit: the raw WASM proposal only really defines how shared memory and cross-thread atomics work where it's assumed each thread

Yes, I should have been more explicit: the raw WASM proposal only really defines how shared memory and cross-thread atomics work where it's assumed each thread runs a wasm module (with shared memory regions that are appropriately mapped.) It does not specify how the host environment actually spawns or manages threads or what hostcalls are available for that. That said I think in the browser something like emscripten can do something akin to "use web workers with shared array buffers" to back it all up so that threading APIs roughly work, but yes, WASI currently has nothing for this.