The article doesn't. It measures productivity in terms of actual monetary value per worker and then compares it to the number of hours worked.
> The effect of automation is very hard to quantify because it tends to displace low-skilled jobs that don't pay much in the first place. But the social impact of this is huge -- you take the people at the bottom of the economy and take away the only jobs that were accessible to them. Social unrest is inevitable. This will likely impact the economy in negative ways in years to come, and we need to understand this.
Again, addressed by the article. Would you have considered milkmen or household servants high-skilled jobs? Both these jobs have been all but replaced for the majority of people, yet there wasn't that much social unrest when the refrigerator or washing machine was invented.