"I had a recent debate over whether a border should be 3, 4 or 5 pixels wide, and was asked to prove my case."
He doesn't give much more context about this debate, but it reminded me of some of my own experiences where I got involved with almost the exact same debate. In my case, the cause of the debate was almost always because there were multiple designers. Either it was two more more designers arguing about the width or it was after a product launch that the next designer decide that the width decision should be thrown out.
What drove engineers nuts was that because decisions are so arbitrary anyone can override the design at any time. Maybe Google is on the extreme end to demand numerical metrics to back up any decision, but for most engineers it was mainly that they wanted to understand the reasoning for a change. Otherwise, engineers would spend their entire lives changing the size of borders.