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.