Here’s another study which found the same thing: http://ehp.niehs.nih.gov/wp-content/uploads/advpub/2015/10/ehp.1510037.acco.pdf
Both studies were published in a top journal in the area (13% acceptance rate) and have respectable university professors in the area among their authors. The reason other studies did not found the same thing is because they weren’t looking at that range. The common assumption was that anything below 800ppm is equally perfect. You will find some blogs in the area giving this same explanation. The effect size really is suspiciously huge, but it is not entirely implausible either given that we already know CO2 does have a brutal effect on cognition at higher ppm and understand the mechanism fairly well.
Anders made the following calculations. A human consumes about 45 liters of oxygen per hour, producing about 10 liters of carbon dioxide per hour. To maintain a good CO2 level you want 8-10 l/s/person in an office. Now, if the airspeed is 0.15 m/s (about the limit of drafty) you need 0.01/ 0.15 = 6.7 cm^2 window area.
My credence on the finding being true is high. Even if it is false, the cost would be leaving the window very slightly opened (6.7cm2 is pretty small), or opening in every other hour. I’ve been doing so since the last study came out and I would add the room also feels more pleasant like that.
10,000ppm causes heavy breathing and confusion, from 50,000ppm upwards you can die in a couple of hours. I don’t think going from 80th to 30th on complex cognitive tasks is totally implausible with an increase from 600 to 2500ppm.
I don’t understand why you find it so surprising given that (it seems) you had no previous knowledge of the area. Where did your prior come from? I can’t be surprised by the fact the Planck length is 1.61x10^-35 and not 2.5x10^-30 given that I had not idea what it was.