Oversimplifying, but quoting from the speech, that particular challenge was overcome as follows: "Thus we muddled through by common sense and compromise to something approaching success."

He then discusses how the problems there led to his thinking on CSP, and, eventually, formal methods.

But, go read the speech, it's much more wonderful than any summary thereof: http://zoo.cs.yale.edu/classes/cs422/2010/bib/hoare81emperor...