WASI is a system interface for WebAssembly modules, it is not an implementation. Therefore, these are quite different.
However, they could have decided to use WASI as the interface for their wasm runtime, but it looks like they didn't
It's likely this project was started before WASI was announced.
a bit off topic, but I was wondering, can WASI API be used directly from c/c++ without compiling to wasm ? Basically to have native perf but still benefit from the crossplatform APIs that WASI will expose
That depends entirely on the implementation (and leaves open the question of ABI and binary file format).
Doesn't WASI defines a sort of ABI (what are the data types, the functions, arguments, return types etc...) ?
Yes, but WASI doesn't define calling convention and binary format, because they are defined by WASM. Your proposed WASI-native would need to define them.