The answer is: it's complicated. Which is most of the time the answer in the WASI world.

For this case it's complicated because some runtime supports https://github.com/WebAssembly/threads which mostly contains things like the spec for atomic but not the actual "threads" specs and then some runtimes (i.e wasmtime) also supports https://github.com/WebAssembly/wasi-threads which is one version of the threads. But a new proposal came into play https://github.com/abrown/thread-spawn so ... it's complicated.