He might say "I had access to AT&T code from such and such sources, and the only constraint upon my access was such-and-such cover material, and later encountered much of the same code without any indication of constraint. Discussions about such matters with colleagues led me to believe that most of them had similar free access to the material".

As said, this same document as an expert report, given his creds is fairly damning ... and repeatable by hundreds, maybe thousands, of others in the industry.  But as a deposition of personal knowledge, I don't think it will stand if challenged.  But ASB it may well not be challanged as IBM can take it out of one pocket (depositions) and put it in another (expert testimony).