That sounds so unlike the YC partners I know that I have to wonder whether you correctly interpreted the response.
The fact that you got a specific message indicates that someone at YC saw potential in your application and was hoping to help you improve it. The subtext of those messages is "help us get to yes".
In my experience they would never say things like "why don't you go back to your day job" or "I don't believe you" or anything in that ballpark. That's just not the culture of how YC treats founders—quite the opposite. Besides that, insulting applicants would obviously go against YC's own interests.