No, he is great source, he just do appear as unreliable and lonely voice because he use Russian sources and not doing the painful sieving grains of truth from the western bog of propaganda like Parenti did (and Parenti still managed to produce controversial books). His problem is that global south marxists are not very interested in delving into history of USSR and even debunking old history in general, they have way to much going with current events, and what he uncovers is very uncomfortable for westolefto and prowestern liberals whose historiography is build upon anti USSR lies.
I definitely agree that the “Testament” is not that big a deal
In isolation yeah, but it is precisely upon a mountain of such "not big deals" that the anticommunism was build. Communists abandoning historiography to liberals was and still is a tragedy.