True, but there is no ongoing guarantee of that, no matter how many times it has happened in the past, and no matter who claims it[1], be it a Midas-list VC or a serial founder or whoever.
Past performance is not a sure predictor of future performance. Just see any company's statutory annual report, and the ifs and buts in the legalese therein.
[1] If anyone claims that, just ask them to put their money where their mouth is. E.g. by paying the predicted unicorn future value for the stock, but now. And then see the disclaimers and caveats flowing fast and freely from their mouths.
Serious question: "Who" exactly are they? I could have told you 10 or even 5 years ago, but now? There are a lot more hands in the cookie jar.
All the group partners and management are people who have been with YC for many many years.
But I don't think there has to be much alignment at all. One unicorn every couple of years is all that YC needs to "make their fund" since they don't have LPs to satisfy. And one every few years will also keep the applicants coming.