It's a description that guns in the right direction.
The Imperium isn't supposed to be a reasonably sensible authoritarian tyranny that does what has to be done to save mankind in a hostile universe, tragic as those necessary actions may be. It is supposed to be utterly out of whack, unhinged and decaying, stagnant and myopic, needlessly murderous and a dysfunctional dead end for human power in the Milky Way galaxy; a failed, impoverished, interstellar empire who can't even do science.
In a galaxy of horrors, the Imperium of Man is itself one of the baleful horrors. That's the true beauty of Warhammer 40'000.
The early Imperium of the Great Crusade was far more of a necessary evil and reasonable dictatorship, a sensible option in a terrible time. It was a post-apocalyptic renaissance and reconquest of the human colonies, rather savage and cruel but still having promise for the future. The Imperium of M40 is a failed promise, the death of a dream, the feverish nightmare of mankind in the darkest of futures.
And the burning of Prospero was pivotal in setting the Imperium of Man on its downward spiral. Since then, the best man could hope for was to tread water to barely keep breathing and delay drowning.
Cheers