It's a top secret report created by the informational gathering apparatus of a global super power/nation state, with all the interest to get an accurate picture of their geopolitical rival, but also with the interest to keep their population not in the know (not it's like the only time in US history). The fact that it fits with other historical accounts of Stalin by e.g Domenico Losurdo.
Funny how you libs always pull out skepticism when it's against the western narrative. Even if it's unvaluated, it's not going to be significantly off. The CIA is pretty good at what they do 