I kinda wonder, if you believe what's missing in your platform is a single feature that can be implemented in 2 days, do you really think YC wouldnt realize that too? So this is likely not the reason for the rejection. I would have even more worries about these founders after this stunt because it shows a lack of self awareness and strategic insight. I'm in no way affiliated but Id guess YC is looking for foundationa…
Also YC even asked to meet them before the next round, so it might have intrigued them.