Like many universities, the value of YC to founders is mostly signalling and network. What you actually learn at YC is a minor component of the value (since you could learn most of it just by reading or watching videos)
As YC batches get larger, the signalling value of being a YC company goes down. If everyone is special then nobody is.
As YC batches get larger, I suspect the value of the YC network also goes down. This is maybe a little bit counter-intuitive, but I think a large influx of less highly-selected startups will dilute the network and prevent it from functioning effectively.
Unfortunately YC is not completely aligned with founders here. There is a strong temptation for YC to increase batch size. YC gets 7% of a company for far below market price. So increasing batch sizes makes short-term economic sense. However, if YC increases batch sizes too much and harms its brand, it will eventually break its own model. So increasing batch sizes could be seen as form of spending down the brand equity.
This all hangs on a few premises that may or may not be true: 1) You can't have larger batches while keeping quality constant. 2) YC doesn't directly influence the success of participants that much. 3) If the value of YC drops, good founders will eventually stop applying
I suspect an argument like this has been going on inside YC for quite a while. And the blog post was written by the original founders to try to settle the dispute in favour of the pro-scale faction.