Well, we certainly didn't clear up the little matter of depth of field at infinity!
The truth of what he was really thinking about, I imagine, is that because the depth of field is so wide at/near infinity, it becomes progressively more difficult for eye or machine to nail the precise plane of focus where it is so dificult to discern subtle differences.
Depth of focus: that 's a measurement that belongs at the film/sensor plane and has nothing to do with anything here, other than the fact that the longer the lens the deeper the depth of focus and the wider the lens the more shallow. You will discover that with your first trip into LF Land, if not LuLaLand!
God, it's hot in this stuffy room. I need a shower, feel a bit hungry but won't eat anything because it's too close to bedtime and I suffer from acid indigestion which threatens to choke me now and again when I forget and eat late. And to think, I used to be able to work until midnight, come home, devour a steak, french fries, peas, whatever else was going, go up to bed, do a hundred-and-twenty to a hundred-and-thirty press-ups and be up again in the morning for breakfast and in time to drive the kids to school. I can no longer do one press-up, I suspect, and am too scared to try. How time mocks us.