Yes, I'm a little skeptical as well. My apartment generally runs between 1000 and 1500 ppm, and I sometimes work outside, but I haven't noticed large differences in ability to work and concentrate between the two. (What does help is moving back and forth.)

I tried keeping my room well-ventilated (500-700 ppm) for a few weeks and didn't notice much impact on sleep and work. There was a burst of positive effects a few days into the experiment but it didn't last and was likely unrelated.

The air definitely starts feeling stale and gross around 2000 to 3000, so I try to keep it below 2000.

That's all personal anecdote, but it similarly drives me to ask: Where's the replication?