(Author here.) When I wrote that, I was echoing what I believed to be a widespread view. For example: https://martinfowler.com/bliki/CannotMeasureProductivity.htm...

"Productivity, of course, is something you determine by looking at the input of an activity and its output. So to measure software productivity you have to measure the output of software development - the reason we can't measure productivity is because we can't measure output."

In an earlier post, I argue that in light of the fact that we can't measure productivity, we should measure job satisfaction instead. https://redfin.engineering/measure-job-satisfaction-instead-...