No they aren't opposing any magisterial document. They are simply asking the 
Pope to clarify a part of his Apostolic Exhortation which is disputed by bishops, bishops' conferences, theologians and academics across the world. The Pope can easily resolve this impasse and has a duty to do so.
It is very easy to condemn these Cardinals but can you cite one instance in the Cardinals' letter where they oppose a magisterial document, just one?