One of the book quotes is about how it is entirely Orthodox to hope & pray for the salvation of all, and says that a number of early Saints believed that. You can't get much more authoritative in the Orthodox church than that. And this book is used for Catechumen classes - It's what some Orthodox churches in America use to teach converts about what the Church believes. The only way to get more authoritative than this would be for something explicit to have been in one of the Seven Ecumenical Councils.