Here's what he wrote about Campbell's Law:
if you don’t think public educators can handle real accountability without resorting to cheating (e.g. the constant refrain of “Campbell’s Law) then you have a pretty low opinion of public school educators. In most walks of life there are high-stakes consequences attached to professional and behavioral decisions. And yet most people are able to play by the rules.
Here's what Campbell's Law actually says:
The more any quantitative social indicator is used for social decision-making, the more subject it will be to corruption pressures and the more apt it will be to distort and corrupt the social processes it is intended to monitor.
I don't know to whom he's referring and what others are actually saying about Campbell's Law, but if he's accurately representing their sentiment then a lot of people are misapplying the lessons of Campbell's Law.
The central insight of Campbell's Law is that it's a really bad idea to make all of our decisions based on any single measure, not that people will always cheat in high-stakes situations (or, for that matter, that tests are inherently bad). It's not about high-stakes versus low-stakes or accountability versus trust, it's about relying on one measure versus utilizing multiple (or no) measures.
So, in short, what we should learn from Campbell's Law is that we shouldn't use only testing data to make decisions in education.