I mentored/advised literally hundreds of startups over the last 10 years, in most cases just out of pure altruism (in a few numbered cases these were startups where I had invested, so the help was not altruistic). You hear "hundreds" and might think it's exaggerated - reason why I interacted with so many startups and they seeked advice was because of the brand I represented (AWS), and the fact that everybody wanted technical help to start with. Let me add: perhaps also because I really love teaching and helping, and it probably showed.
I've seen many other mentors/advisors during these interactions, and most of the time the truly helpful ones were the ones with real experience and a real desire to help; their ability to help came mostly from their experience (sometimes successful; but surely painful and full of great life and professional lessons).
They say you can't teach experience, and in this particular case I see this as the biggest problem that not even YC might be able to solve or mitigate.
If you think that an initiative like this can "do no harm", I think you're mistaken: when too many want to advise/help, but don't realize they lack the true experience to know what they're talking about, the risk is that they'll simply damage these startups.
To be clear: I am in favor of what YC is doing here; I am simply sharing my views on what the biggest hurdle can be.