That said, I see how it can apply to changing monsters every so often, but I don't see how it correlates to attrition per se.
Care to elaborate?
It's not related in any direct way. That was a comment about ways to figure out how long is long enough. If you set the target at 30 minutes and a battle lasts 2 minutes, then you should aim to have at most 15 encounters before progression. That can then be used to figure out the dungeon length, encounter rate and encounter danger level.