Capilaries don't have to be one dimensional. Capilaries just need to be narrow enough.

Honestly, the wording of this entire thing is a bit forced. IMO the word "capillary" is suggesting that surface tension has anything to do with it which probably does not. They also use "low humidity" to describe a place in the 2d channel where there are other particles. For me "humidity" is a statistical concept and is also supposed to be about proportion of water vapour and air.