Comment by bluetech

> Object.freeze does make an object immutable

If you look at the Hardened Javascript example:

    const makeCounter = () => {
      let count = 0;
      return harden({
        incr: () => (count += 1),
        decr: () => (count -= 1),
      });
    };

The "counter object" here is mutable. I think that's what they mean.