If you re-read the opening paragraph on that page (470) it mentions "You can only call methods on an anonymous inner class reference" here is the important part "that are defined in the reference variable type!"
So in this example the reference variable type is Popcorn and p is the reference variable. So because sizzle() is not defined in Popcorn it can't be called polymorphically (I feel so smart using the word polymorphically ) So sizzle() must be defined in some way in Popcorn class for it to be used. The whole chapter is worth re-reading to secure differences between Inner classes and anonymous inner classes, and how to reference the methods and variables, etc...
The first person to drink cow's milk. That started off as a dare from this tiny ad: