Suppose x and y are exact numbers. Then x+y is represented as the open interval (x, x+ULP) where x+ULP is the smallest representable exact unum greater than x. Since x is disjoint from the open interval (x, x+ULP), it satisfies the inequality.

Folks, if you don't want to buy the book, Amazon lets you do a "look inside the book" that gets enough introduction to explain the unum format, for free.