These stats are pretty dubious. I can hit the "I believe" button for some, like a 23% drop in electricity costs. One day in five not in the office... sure, a one in four drop in electricity is close. But a drop in paper usage of over half? That's either not accurate, or it's not attributable to taking Friday off. It could be "we cancelled the meeting on Friday where everyone brought a ream of printed materials (that no-one read)," but tying that to a day off isn't believable.
The more important number is the near-40% increase in production. And the reason it's not believable is they're going to try a four-day workweek again, but only next summer. If I'd tested the workforce and got a 40% increase in production by giving them a day off a week, I'd keep the change permanent. Right away.
Now, why wouldn't they make it permanent immediately? Well, it could be labor laws, which make all kinds of things illegal. Or it could be that they really think it's something else, like the fewer meetings that the article mentions. But it stretches credulity to say that you can get a 2/5 increase in production while dropping hours by 1/5, and the company doesn't make that change permanent, immediately.