Their claim that

    let s1 = String::from("hello");
    let s2 = s1;

    println!("{}, world!", s1);

is in fact safe code would suggest they indeed don't have much concern for thread safety.

You aren't missing anything. It's a good point.

We're beginning with what is central in C and then moving outward. C, as designed by Ritchie, and for the first 40 years of its existence had no official support for multithreading, so this can hardly be called central.

If we add support for multithreading it'll be after finishing with C89 and C99.