Adding CO2 will always decrease the pH. That actual pH depends on the KH, but the change in pH with added CO2 is always a reduction.
That is what I thought as well.
The carbonic acid concentration decreases so as to bring the system back into equilibrium with the new lower level of CO2.
While the rest of the quoted material is fine, this is the important part. When there is a lower level of CO2 (i.e. CO2 is no longer being injected, or a lower amount is being injected) present in the water column.
With less CO2 being injected, the pH will not decrease as much, resulting in an "increase" in pH (rather, the pH shifts back closer to the initial value before CO2 injection).