This is really interesting and contains a logical fallacy.
The interesting part is that Fred Brooks' "No Silver Bullet" may have in fact been wrong. The silver bullet turns out to be language design. A 2007 programmer can code something in a few minutes that would have taken a 1972 team a few weeks, and the reason is that languages have evolved considerably since then.
The logical fallacy is that the productivity gains are averaged over the intervening years, despite the fact that the language used to demonstrate this has itself been around since 1995.
You can say we've got an 11x increase in productivity since 1972 due to language design. You definitely can say that, although don't blame me if you do say that and the Lisp people immediately descend upon you and eat your head. But you can't say that productivity doubles every six years because you don't have any data on the trend that indicates the continuance you're implying or the consistency you're implying.
Very interesting, but logically unsound.