Why ringbuf crate need mutable reference

Every time I search for "ringbuf" in lib.rs, the ringbuf crate always comes up first and it claims to be lock-free. When I take the time to read its documentation, I find that modifying it requires a mutable reference. I don't quite understand why this library claims to be lock-free when it clearly doesn't support multithreading at all. If it did support multithreading, why would modification require a mutable reference?