Although the paper's title and abstract seem to be intentionally trying to stir controversy and annoy people, the core issue of FFI unsafety is real.
C has many subtle behaviors, and a type system that doesn't describe them well. It also can't express its API's ownership or thread-safety, and there's no true immutability. Correctness of the ABI isn't enforced in any way, so you just smush symbols together and hope for the best. That is ripe for errors, especially when you try to add extra guarantees on the Rust side that C may or may not actually provide.
C has many subtle behaviors, and a type system that doesn't describe them well. It also can't express its API's ownership or thread-safety, and there's no true immutability. Correctness of the ABI isn't enforced in any way, so you just smush symbols together and hope for the best. That is ripe for errors, especially when you try to add extra guarantees on the Rust side that C may or may not actually provide.