"Yes, you can measure software developer productivity" - But should you?
Probably not.
McKinsey wrote about developer productivity here.
The central inner/outer loop argument in the article is flawed because it perpetuates the stereotype of developer as code machine.
Coding is a creative activity and the ratio of thinking and collaborating to writing can have a wide spread.
This is what makes it (or any creative knowledge work) difficult to measure, especially on an individual basis.
A lot of the push for productivity measurement and the general tone of this article is that leaders just need to measure enough so they can figure out what to fix.
The industry is steadily moving towards subjective measures and the DevEx Framework is a good example of this evolution.
Turns out that if you just ask people what is holding them back, they will tell you. If you work on clearing those things away, your business measures will move in the right direction.