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.
> I have been unable to find a problem that breaks unum math.
Then perhaps try (x+y) != x, where x is a very large, and y is a very small positive number.