This factoid has been proven false multiple times, in multiple forums over the last year. Its incredible that a CEO of a company that claims DNS expertise wouldn't know this was false. One particular "internet security" company was PINGing the root servers, and some of the root server operators turned off ping. The root servers themselves were unaffected (except maybe one operated by the US Military).
It might be a matter of interpretation. According to http://d.root-servers.org/october21.txt: 2.1. Some root name servers were unreachable from many parts of the global Internet due to congestion from the attack traffic delivered upstream/nearby. While all servers continued to answer all queries they received (due to successful overprovisioning of host resources), many valid queries were unable to reach some root name servers due to attack- related congestion effects, and thus went unanswered. While I'm not trying to act as Sclavos' apologist, I think you have to be careful about how you respond to this particular claim of his. You can't dismiss it out-of-hand. Misleading? Yes. Flat out false? You'd have to be more convincing.