As someone who went through YC and met PG while he was still actively running it, I can tell you that at least in our batch he was 100% spot on about who was going to do well. He had a tendency to spend his free time with the same individuals who ended up doing phenomenally well. It would be easy to be dismissive about this and say something about doubling down on his best investments, etc. But during our batch many of those companies had not yet become the clear cut winners that they are today, and instead only turned into them a year or so down the road. One of the best companies from our batch is Flexport, and PG had been super impressed by Ryan Petersen even before he was officially accepted into YC [1].
It's pretty clear in hindsight that taking note of who PG was hanging out with was a very investable strategy. In fact, one of my batchmates had some money to put into other companies in our batch, and when he asked me for my opinion of who to invest in, that's exactly what I told him. I just did some quick math and would estimate that his ROI today is somewhere in the 1,000x range.
I am not the only who figured this out - while this wasn't quite as much of a factor during my batch, nowadays there is an entire cottage industry of YC-only funds who invest only during YC demo days. As you can imagine, the way they differentiate from each other is by how much insider knowledge they are able to accumulate leading up to demo day (I am not implying that the partners ever leak anything, but these funds are very motivated and they look for all sorts of signals). One such signal is to track all other investors around you, and a well-connected angel can single-handedly increase the valuation of a startup (which explains why they manage to get in before demo day and at a discount).
I have to say though - while the success rate of these YC-only funds is likely good enough to make them quite profitable, none of them come even close to what I observed with PG's ability to pick the winners (which makes sense, since a lot of other people have tried to build accelerators and none of them come even close to YC).
[1] https://youtu.be/syoqjYLDs48?t=1169