The parent comment is about adjusting pay for productivity, what is the reasoning behind "in a world where you compensate for productivity, you'll have to pay more employees that are less productive"?
The other reason is that pay transparency is often anchored to specific credentials and characteristics, and it's often the case that credentials can be orthogonal to work performance. You might have a PhD who can't code who demands higher pay than a HS dropout who designed your entire infrastructure, and the HR department (or lawsuits, or etc) would force you to comply.
Such concepts punish high achievers.