It's an interesting comment by Joyce. But even though Joyce may have played Agassi and Sampras a few times, he wouldn't have seen them at their best. If Joyce got to a Wimbledon semfinal like Todd Woodbridge in 1997 for instance, he would have seen a different Sampras. I would assume neither player needed to make too much of an effort to beat Joyce or bring out their best shots.
So the comment for Rios' book is designed to make Rios look good for his fans who will buy the book. Not sure how many would take the comment too seriously however.