Nobody said that at all. Air filters can't stop every particle of dirt in the airstream from entering the engine.Some particles will get through. There can also be small leaks in the ductwork and around the airbox that allows air to enter. More dust in the air more chance of it entering the engine.
Yeah, to have less air restriction with a filter of the same area, the media will have to have larger pores - thus a decrease in filtration efficiency. You could decrease the pore size to increase filtration efficiency as well as to lower the maximum size of the contaiminents it will pass, but will in turn need to increase the area of the media so as to maintain air-flow efficiency.
In use, media pores will plug and the filter will become more restrictive, adding to pumping losses/decreasing the engine's volumetric efficiency. Transmission shift points will also rise slightly because of the required throttle advance that is needed to overcome the filter's airflow restriction. All this has an impact on fuel consumption, transmission longevity, and most minimally oil/fluid temps.
Driving in dusty conditions will present fine airborne contaiminents to the air filter system, and increasing filter loading as well as the sudden inrush of finer contaiminents whose size is likely too small for the filter's rating. Not all that passes the air filter will make it into the crankcase oil, but rather out the exhaust as some have said around here.