Well, I like the story here, but it's kinda against a bit of a strawman. Don't get me wrong, I'm not losing the overall point of the piece, a point I agree with, but that said a metrics focussed manager could have simply added an "adjunct" label to the stories and had this "worst programmer" add themselves to stories as the non-lead developer. Ultimately the best way of measuring programmer productivity is by the ass…
Maybe ... Productivity itself is difficult to define. Different people will value aspects of the work differently
At one Internet Advertising company I worked for, the founder had written most of the original code. Written in the late 1990s, it was Perl, JavaScript, HTML, and SQL jumbled together. Completely ignoring the notion of 'separation of concerns'. Huge amount of code duplication. Source code control? Phhht! Nary a test in sight. Ran programs as root to work around permission problems. Self modifying code ... you betcha. He worked right on the production servers. He could and would push out a feature the same day a customer asked for it. VERY quick to make the customers happy. The company was being bought out at the time I came on board. Pocketed his millions, headed down the road, yay for him.
My own take is that the company would never have 'made it', if a software team had been hired to 'do it properly'.
After his departure, as we rewrote our code base to 'professional' standards, much time was spent refactoring that produced few or no new features. How productive was that? A very complex question.
As a digression, I sometimes found his original code easier to maintain that the stuff done 'the right way' because there WAS no 'separation of concerns'. I didn't have to hunt in a different part of the source tree for where something was done. It was all 'right there'. YMMV.
In the end, 'productivity' is way more subjective that we'd like.