He says that in writing the book the author may have gained fame in the short run, but at the expense of surrendering his honour.
I like that he uses that argument. Honour is usually disregarded, but it is important.
Some would say that Matthis' own duty is to the country and that staying silent about a commander-in-chief that he knew was doing damage to it is a decision which itself is dishonorable. Thereby, he is placing a burden on his subordinates who know the truth and are uncomfortable with Matthis' silence. In this sense, Matthis standing on his silence as the only honourable course is more about vanity than responsibility.
Sometimes there are competing claims upon a person's sense of what is the right course of action. It sounds like Matthis doesn't know this important thing.