[islon]

I don't think the author is saying that there's no difference between productivity between engineers, it's exactly the opposite, there are so many differences and nuances that it's almost impossible to measure all of them. Your mood, chair, coworkers, boss, temperature, programming language, editor, task, etc. all have some influence over your productivity. You can't say "all things equal" because things are not equal in the real world. That's not to say that performance measure is hopeless but that it's as complicated as weather forecast: you have a huge ammount of variables.