In my experience, lack of proper ventilation is a much bigger problem than background CO2 levels.

In other words, this article only holds under the assumption that ventilation stays as bad as it is. If we improve ventilation, we can get an improvement much bigger than the increase in background CO2.

The article even says so:

"In a 2002 study, researchers tested the air in 120 Texas classrooms, and found CO2 levels above 1,000 ppm in 88% of them. In 21% of the classrooms, the concentration exceeded 3,000 ppm." -- current background is ~410 ppm (possibly higher in cities) so even a doubling + better ventilation would improve the situation in most classrooms.