Laurie, Joyce did not see Rios at his best either. Rios was at his best at the Australian Open 98 in his matches before the final and Miami in 98, as well as various other matches in 97 98 99.
Also, I don't think the quote by Joyce was designed to sell the book, he simply had a very high regard and respect for the unusual talents of Rios.
"In the beginning my coach was angry at me because I was too nice on the court." --Gustavo Kuerten