>>> Which is not necessarily productivity

True. But, while I can't access the article, I suspect this might be an "all other things equal" estimate. People are always saying how they used to have so many interruptions in the office, and now they don't. But that's probably taken into account here. So maybe an 8 hour day is 6.5 hours of work in the office, but 4 hours at home.

Even if it's not really 2.5 hours, I think it's inevitable that most people work less if they can get away with it. People who are highly motivated and love their work might not have that problem, but they're the minority especially in big companies.