How does an IIFE's being called immediately prevent it from polluting global scope?

In a Udacity lesson on immediately invoked function expressions (regarding the provided code snippet) it says:

The function that is being returned closes over (i.e., captures) the hi variable. This allows myFunction to maintain a private, mutable state that cannot be accessed outside the function! What's more: because the function expressed is called immediately, the IIFE wraps up the code nicely so that we don't pollute the global scope.

I'm strugggling to understand what calling the anonymous function immediately has to do with prevent the variable hi from "polluting the global scope," and since hi is already defined in a function, isn't it already in local/private scope?

const myFunction = (
  function () {
    const hi = 'Hi!';
    return function () {
      console.log(hi);
    }
  }
)();

In modern JavaScript, you have let and block scope (and I'm pretty sure const has block scope, too), so you could just do this:

let myFunction;
{
    let hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
}

This creates myFunction without leaking hi into the surrounding scope.

In traditional JavaScript, where you only have var and function scope, you could do this:

var myFunction;
function a_private_scope() {
    var hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
}
a_private_scope();

a_private_scope limits the scope of hi, but (it being a function declaration) it needs to be called explicitly, and we still leak a name to the surrounding scope (this time it's a_private_scope, the name of the function-serving-as-a-scope).

By using a function expression and immediately calling it, we avoid this second name pollution:

var myFunction;
(function () {
    var hi = 'Hi!';
    myFunction = function () {
        console.log(hi);
    };
})();

Now the only thing defined in the outer scope is myFunction. The anonymous function that serves as a scope for hi has no name it could pollute the surrounding scope with.

Finally we can clean it up a bit by using return values, so we don't have to mention myFunction twice:

var myFunction = function () {
    var hi = 'Hi!';
    return function () {
        console.log(hi);
    };
}();

(This also saves us a pair of ( ) because the function keyword doesn't appear at the beginning of a statement anymore.)