I don’t think they were really criticizing a cardinal, but simply stating the traditional Catholic positions. Now Kasper, on the other hand...
Maybe the five cardinals intended it as an academic exercise/exploration of divorce and remarriage at the communion table, but once their publisher got ahold of it - hoo boy! - it got repackaged as criticism. This comes from the official description of the book, found at the Amazon listing for the title:
In this volume five Cardinals of the Church, and four other scholars, respond to the call issued by Cardinal Walter Kasper for the Church to harmonize "fidelity and mercy in its pastoral practice with civilly remarried, divorced people".Beginning with a concise introduction, the first part of the book is dedicated to the primary biblical texts pertaining to divorce and remarriage, and the second part is an examination of the teaching and practice prevalent in the early Church. In neither of these cases, biblical or patristic, do these scholars find support for the kind of "toleration" of civil marriages following divorce advocated by Cardinal Kasper.