How does this evaluate to true?

I am reading PHP Cookbook, in the section about Forms there is this:

Problem: You want to make sure a value has been supplied for a form element. For example, you want to make sure a text box hasn’t been left blank.

Then the author elaborates to this:

// Making sure $_POST['flavor'] exists before checking its length
    if (! (filter_has_var(INPUT_POST, 'flavor') &&
        (strlen(filter_input(INPUT_POST, 'flavor')) > 0))) {
        print 'You must enter your favorite ice cream flavor.';
    }

I don't understand how this 'and' expression evaluates to true.
Won't !(filter_has_var(INPUT_POST,'flavor') always be false?