I think it is pretty easy to illustrate why fewer tests are needed in certain statically-typed languages.

JS

    function max(a, b) {
      if (a < b) {
        return b;
      }
    
      return a;
    }
    
    console.log(max(1, 2));
    // 2
    console.log(max(2, 1));
    // 2
    console.log(max([1,2,3]));
    // [1,2,3]

Rust

   fn max<T: PartialOrd>(a: T, b: T) -> T {
       if a < b {
           b
       } else {
           a
       }
   }
   
   println!("{}", max(1, 2));
   // 2
   println!("{}", max(2, 1));
   // 2
   println!("{}", max([1, 2, 3]));
   // Compiler error

In JS, it would be prudent to test for reasonable misuse of this kind (return undefined, throw Error, etc). The author claims _This is a dynamically typed language. You get exceptions when you mess up the types._ That isn't true in this case and it is not clear from the code what would/should happen.

In rust, you don't need any tests like this.