Sorry, sloppy, the point is that the lighter gas H2O(18) rises in air while the heavier gas CO2(44) sinks.
The reason water vapor rises is it is lighter than all gases, not just CO2. CO2 doesn't sink to any appreciable extent (e.g. the readings at the top and bottom of Mauna Loa are continuously compared and match pretty well). When water vapor rises it is part of the cooling process, transferring latent energy upwards where it is released with the water vapor condenses.