I actually think this is a pretty general problem with “great books,” for reasons that are explained in Thomas Kuhn’s The Structure of Scientific Revolutions which is, itself, a great book that suffers from the very same problem. Obviously part of the issue is simply that there’s no guarantee that conceptual innovators will be good writers. But the deeper Kuhnian issue is that great game-changing thinkers end up altering the conceptual terrain in a way that renders their original works obsolete-sounding and confusing. Meanwhile, a whole discipline grows up in the shadow of the great book and its practitioners develop a nice clear reconstruction of the framework.
But the availability of these clear reconstructions only makes the original look even worse. If that’s what he meant, then why didn’t he just say that!