The point here isn't that nothing can affect the curve, but rather the nature of the curve.
I don't think his implied analysis quite makes sense – if you are doing something that is OK delivering 20 people at D30, and you can decrease attrition by 10% from D1 to D30, then you get 26 people at D30. If you decrease attrition from first visit to signup by 10% you also get 26 people at D30, even though you've "gained" 80 people that have signed up. Except for some reason the math would lead to 28 people at D30 (D30=Signup/10), for reasons I haven't figured out. I guess because you lose 4 in 5 people from Visit to Signup, but you only lose 3 in 4 from D1 to D30.
All that presumes that it's somehow just as easy to effect a 10% improvement one place as another, which is unlikely. What is likely? Not sure.