My feeling was that this rewrite is not actually "done". Sure, all their C++ has been converted to Rust, but it seems like there's a lot of unsafe that they could rewrite in safe Rust.

And for the C libraries they vendor in, assuming none of them are exposed directly in their public API, it's likely they can replace them with Rust libraries with equivalent behavior. mbedtls seems like a good example of that; certainly it wouldn't be a small effort to switch to rustls, but it might be worth it to do so. And even if they didn't choose to do that, I just did a quick search on crates.io, and it looks like there are safe wrappers for mbedtls.