The difference in terminology depends which edition of the book you read. In the second edition he includes a long passage explaining that a lot of readers of the first book misunderstood the word "affordances" and so he was now introducing a new term, "signifier," which means what people thought "affordance" meant.

In short, the app already affords you the ability to scroll to the right to view more options, but there's no signifier telling you this.