Maybe, YC is trying to teach you something about product building. Something like: "we don't make our site mobile friendly because the content is so good that people still use it on their phone even if they have to zoom in a little. The day when people stop coming to hacker news or apply to YC because it's not mobile friendly, then we might do something about it. Otherwise it's just a waste of time and money for us." In other words: don't rush into building everything people ask you to do, they might not really need it.

It's probably not what they intend to do... But still worth thinking about ;)