This article seems to conflate "works less than eight hours a day" with "doesn't get the job done." I would expect somebody who doesn't have the distractions of an office or the pressure to appear busy for eight hours regardless of actual workload might get things done faster.
Also, a lot of telecommuters are either intermittent workers (e.g. freelancers) or part-timers, so it's unsurprising that a significant number don't average many hours a day.
Interesting, I would say depends highly on what you are doing, and how self motivated you are. With certain tasks, I feel a day in the office is as productive as a week from home. I also found it weird that everyone jumped on the efficiency bandwagon. I actually know people who work from home, and there are definitely a sufficient number of them who do very little productive work in a given day (not that it is necessarily different in an office).
Also, a lot of telecommuters are either intermittent workers (e.g. freelancers) or part-timers, so it's unsurprising that a significant number don't average many hours a day.