Some of the organic material breaks down really slowly, providing some nutrients to bacteria at a rate that allows the gas to escape, but yea essentially the trick is the ultra high surface area and high compressive strength so that it maintains a porous structure instead of compacting into an anaerobic environment.
Yes, beneficial fungi reach much deeper than roots due to the higher penetrative ability of hyphae and they can still extract stuff from whatever depth they reach.
Long term I want to be able to do what the guy in the video is doing but with out all that shoveling...