It's a little bit more than pure Rust: to build the library there's basically only two dependencies (rayon and wide) which bring only 14 transitive dependencies (anyone who's built even simple Rust program knows that this is a very small number).
And there's more, Rayon and wide are only needed for performance and we could trivially put them behind a feature flag and have zero dependency and have the library work in a no-std context actually, but it would be so slow it would have no use at all so I don't really think that makes sense to do except in order to win an argument…
As in you don't need to have C compiler, python, dynamic libraries. "pure rust" would be a better way to describe it.