I hadn't thought of certainty as one of his shortcomings, though I can see why you say it was. It made me think of this passage from his autobiography, where he too criticizes certainty while drawing a distinction between certainty and conviction. (I transcribed this from the audiobook, so punctuation might be wrong).

"Over the course of the last decade I've become vividly aware of a literally lethal challenge from the sort of people who deal in absolute certainty and believe themselves to be actuated and justified by a supreme authority.

To have spent so long learning so relatively little, and then to be menaced in every aspect of my life by people who already know everything, and who have all the information they need.

More depressing still to see that in the face of this vicious assault so many of the best lack all conviction, hesitating to defend the society that makes their existence possible. While the worst are full to the brim and boiling over with murderous exultation."