The higher the average price you pay, the fewer deals you can afford to be in
That's only true if you have to get a certain amount of stock, which is the case for VCs doing series A rounds, but not for angels or VCs doing seed rounds of the type Chris wrote about. How many deals an angel or seed investing VC can afford to be in is a function of the average amount invested, not the average valuation.
When a startup is crazy successful, it doesn't matter what price you paid to get in. But in terms of portfolio, price does matter. The higher the average price you pay, the fewer deals you can afford to be in, and the lower your chances of getting into that one runaway success. Sure, its far more important to choose the right companies. But it is rational for YC, and others, to be price sensitive.