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