I went back to read this portion[1] and it actually says, "If we get 1000 applications and have 10 days to read them, we have to read about 100 a day." So take it in context: PG is really trying to encourage applicants to put effort into their applications so that they stand out. And that doesn't mean to write a ton in your application but instead write concisely enough to answer the question well.
I'd also go out on a limb and say that this is a subtle message to applicants to apply as early as possible. PG is admitting that it is indeed difficult for the partners to keep their focus on so many applications throughout this period. Applying earlier gives the partners more time to review our applications and hence, our applications get reviewed thoroughly and they have more time to engage with the applicants (if interested, of course). Essentially, a win-win. Now that I briefly inferred/annotated this part, I definitely wish I had read this section more carefully. I resubmitted my application several times all the way until the day it was due.
Anyway, as a follow up to the posted question, how do you manage to continue advising YC alumni while reviewing so many applications? Especially the recent class (YC W13) that is probably meeting with tons of investors right now and needs the partners' guidance.
[1] How to Apply to Y Combinator: http://ycombinator.com/howtoapply.html