The claim that Rust makes the safety of 20% of code worse requires some justification. It is utterly contradictory to my experience. Having to do a dynamic check (an array bounds check, or `Option::unwrap`) does not make code "less safe".
This post makes a cogent argument that Rust's rules that are normally enforced by the compiler makes unsafe rust harder to write without UB than other languages that are not memory safe by default.