Clearly his reasoning is bogus: the downstream of a downstream contributes to its upstream, which is your downstream, so you can't tell its contributions from your downstream. In other words, Ubuntu is probably contributing back to Debian (though there may be gripes there too - but that's a separate issue), and the metrics he's gathered have limited visibility into such contributions.
Another interesting metric might be changesets per group-member. 'per capita' if you will. How may employees to Red Hat, Novell, Wind River and Canonical each have? How many developers do Debian, Gentoo, Mandriva have?