new Boolean(false) returns an object. All objects (except document.all in browsers) are truthy.
As a result, ! of any object will always be false.
To prove it to yourself, you can run this in your JavaScript console:
(typeof new Boolean(false)) // "object"
Also, you can use the strict equality operator === to confirm that new Boolean(false) isn’t really false:
new Boolean(false) === false // false
Incidentally, calling the Boolean function as a function—without the new—actually does return a primitive:
!Boolean(false) // true
(typeof Boolean(false)) // "boolean"