Yes there is some glue needed. The amount of boilerplate needed can be reduced with a bundler or depending on the library you use (just like it is the case when you use workers with JavaScript on the web, you can use a bundler with support for them or plugins to make it less of a pain to setup). wasm bindgen rayon provide support to use rayon in that context. There are also projects that provide lower level apis similar to std::thread. Std::thread will probably never support the wasm target.

It’s not WASI that gives you access to threading. Multi threading is a WebAssembly spec feature. Each thread is a separate web assembly module, they can share memory/code using SharedArrayBuffer if needed or WebAssembly tables for code. The host runtime for WASM whether it’s the web or something like wasmtime is responsible for instantiating those module in Web workers on the Web (or node…) or in new OS threads for wasmtime. I guess this is the thick amount of glue needed