I don't find it idiotic at all. Even Horowitz himself would be unable to know how deeply he has been influenced by his Stalinist past, and how much of it continues to perdure in his intellectual approach to issues.
And it's not "guilt-by-association" when the featured Trotskyite is proud of his past, and more than happy to proclaim it to the world.
Why is that? Why is it that in order to be a critic one must assume that one knows the author better than he knows himself?
Frankly, I find the whole business of deconstructing motives to be a hinderance to the formation of any intellect at all.