Here is a link to the actual study (PDF via GDrive)

One of the authors of this paper is from the Chicago School and the Hoover Institution. Both are pro-business, anti-worker think-tanks that have been this way for decades. They also don't do any research of their own, but cite other papers that show the 5-20% reduction.

However, the methodology mentioned in the papers is suspect. First, they show that remote workers have the same productivity, but work longer hours. So the net output doesn't go down, they just spend more time working. Which raises the question: How many more breaks were they taking throughout the day? Being remote means a much more flexible schedule, so it's not uncommon to take longer breaks if you're a salaried worker.

Another study was IT professionals shifting to remote work at one company at the start of the pandemic. This one showed an 18% reduction in productivity. But considering the timing of this and that company culture and procedures can contribute to this, it doesn't seem to be a valid data point.

Then they bring up some common criticisms of WFH, which I've seen and refuted since I started working from home 2009: People can't communicate, working in groups is harder, and people can't control themselves. Yawn.

Honestly, the fact that they cherry picked hybrid work as being equally productive shows me this isn't about productivity, it's about keeping offices open. Which makes sense considering one of the authors is affiliated with groups that want to prop up the commercial rental business.