promisifying an already promisified function with promisifyAll

According to Bluebird documentation about promisification:

If the object already has a promisified version of the method, it will be skipped.

In fact, the promisifyAll() also creates an Async version of an already promisified function, which is expected to be:

idempotent in the sense that promisified functions are instantly returned back.

As pointed in Issue Idempotent promisify #159

So in the following example, I was expecting that both obj.foo() and obj.fooAsync() have the same result.

let obj = {
    foo: function(){
        return Promise.resolve('I am foo!!!')
    }
}
obj = Promise.promisifyAll(obj)
obj.foo().then(msg => console.log(msg))      // > I am foo
obj.fooAsync().then(msg => console.log(msg)) // prints nothing

Which part am I misunderstanding?

Running example here

via Miguel Gamboa