The logic arguing for this setup I’ve seen is:
1) The whole point of a CO2 scrubber is to lower the CO2 in the tank water below that which you would otherwise have with the tank equilibrating to the CO2 in your room air.
2) Your skimmer, with the very high and dynamic air bubble production, is a large contributor to total gas (CO2) exchange between room air and water, so a good target.
3) If the scrubber is working to lower CO2 level in your water, that means the CO2 level in the air inside your skimmer cup will have a lower CO2 level than room air.
4) Less media will be used to extract CO2 from this low CO2 level than it would if you started with room air (less media consumption for same target low CO2 level). Without this, the skimmer cup air with low CO2 would just mix with room air eventually.
5) The humid air issue makes sense, and may be part of it, but not what I’ve seen cited as the main reason for this specific approach as opposed to just have a small amount of water in the canister or drawing air from inside the sump cabinet (which is humid).
John -- do you recirculate through the cup?