That's supposedly WASI, an interface specifically designated for system programming use, and that's where it implements part of the POSIX support including pthread.
OTOH you still need to start a wasm runtime first, then import the WASI module into the wasm host.
P.S.: used to tinker with wasmtime and wasmi to add wasm support to my half abandoned deno clone ;) I learned this the hard way
WASI does not implement POSIX. And it isn't the goal of WASI to implement POSIX.
It does not support pthreads either. WASI is similar to POSIX because its an API that provides a "system interface" for WASM programs. But the API is much simpler and quite different and capability based.
There are things like WASIX (by Wasmer) built on of WASI that aims to provide POSIX support for multiple languages
OTOH you still need to start a wasm runtime first, then import the WASI module into the wasm host.
P.S.: used to tinker with wasmtime and wasmi to add wasm support to my half abandoned deno clone ;) I learned this the hard way