But the 1/(1-n) equation is still true no matter how many alternatives you have. The only change is if you have an alternative, say 10% for 1M from some VC, then you would ask yourself after I got 1M and the help VC, would the investment from YC still help my startup grow 6.4%?

I think you are missing my assumption in my OP "in the early stage of a startup". In fact at that stage of startup, even if you have a better deal than YC, you might still want to do YC (so you would take both two deals) because even after your take the better deal, YC can still gives your startup >6.4% growth.