it's not a guarantee, but it's very often a correllated indicator.

Not the same thing at all. Most funds just follow the stock market's performance. They are at the mercy of the market fluctuations. What this investment is saying is that Yuri believes strongly that YC has the ability to pick winners more often then not. And they do it often enough that it's not even worth his time to do anymore diligence.

He is saying that if he spent a bunch of time on each company the odds of him doing better then YC has already done is not worth the extra effort. He is betting he can get a good return by just trusting PG and company's judgment. Pretty awesome endorsement, imo.