At the end of the day it's YC's call and really none of my business. But every now and then I get on my soapbox...
The choice between the two is not nearly as obvious as you think. Otherwise we should all play the lottery, no? The reasons why a is a pretty good idea can get pretty nuanced and sophisticated, and whether a 20-year old knows them all has more to do with who he's talked to recently than any actual ability.
The only reason I'm in a startup now is that I read a few of PG's essays many years past the age of 20, and started having conversations like this on reddit and news.yc. I think it's worth the minor investment in time/effort to reach out to me/them if you're a seed firm.
The other person does not have all the data that I have and probably has not projected future possibilities of the optimal outcome. In that case, they may equate a startup in the same neighborhood of probability as a lottery win.
The ad makes it clear that there are people who will be willing to help you in if they think you've got what it takes, so that it's clearly not as closed as it might seem otherwise.
So rather than try to second-guess why every source of people (GOOG offers to Stanford) is unlikely to give you what you want, give every source the chance to stand up and grab an opportunity.