Software is a form of literacy - and we measure literacy completely differently. In fact we measure it like we measure science - you are not a scientist unless other scientists agree you are, and you are not a coder unless other coders say you are.
What Fowler wants to measure is not the top echelons of productivity but the lower bounds - presumably to winnow out the unproductive ones.
But that is not how we conduct ourselves in literacy or science. We educate and train people for a very long time, so that the lower bound of productivity is still going to add value to human society - and the upper bounds are limitless.
What Fowler is asking for is a profession.