I don't think we've stopped trying to measure productivity, but when the author says "theres no way to measure productivity" he's short circuiting a whole meta conversation about trying to measure if a monorepo is better than a multirepo approach because the merits of the measurements then become what the argument is about and you end up with the same place but with a level of abstraction on top of what you are trying to determine.

Lots and lots of other industries struggle with measuring productivity. The medical profession is rife with that problem. I think what might exaggerate the problem in software is we over value the objective decision making. Its our nature to want to quantify and we don't like making subjective decisions even when that is what we are being called to do.