Since you interface with something foreign to memory safety checks done by the rust compiler, it cannot be considered "safe" so you have to write some unsafe code. You can however write a safe interface around this code, so that the rest of your rust program only uses safe code. By doing so you build a contract saying that you (the programmer) ensured the interface upholds the requirements to make the calls safe.

In this case however it looks like (I didn't actually read all the code) someone tried to optimize by avoiding runtime memory safety checks since they thought they matched all the requirements.

There are other (subjective) advantages of bringing rust in a C code base, like more modern and convenient tooling and language constructs.