The "Why" section exclusively opens with a reference to zstd-rs:
Why though?
Using zstd in Rust is already supported via the zstd crate, so why bother with a whole new implementation?
Separately, I'd say their end goal seems to be replacing the C library dependency in Rust's zstd-rs:
We have our own fork of the zstd that uses libzstd-rs-sys instead of the C library. We'd like to upstream this at some point.