OK, I'm beginning to get this now. It seemed odd to me that carbonate would be more soluble in water made acid by CO₂, thus increasing the concentration of the dissolved CO₂ species. But actually the point is that calcium ions are more soluble in water made more acid by CO₂. My mistake was think of generic "carbonate" when in fact it is the calcium chemistry that is important (and apparently also borate according to kzb).

kzb: 2NaOH + CO₂ = Na₂CO₃ + H₂O
was what I had in mind.