Also even if you have a cofounder - it really matters for YC what your guys accomplished, strong technical experience doesn't matter much, i believe they are looking for stanford\mit grads or somebody with a strong\successful business experience so that they can make a safe bet.
no sane developer would trade his idea he spent several months working on for just 15k, even if it's YC. That's not how investor's market is right now.
Yes single founder is definitely an issue for them,
However I am surprised they didn't even invite you to the interview. Are you currently in US? Could be a reason.
Anyway looks like you are doing great, if you ever want to do something together - let me know, I'd love to know more about how many people you guys have and for how long have you been operating etc.
I am getting a sense that you either should have a strong founders team or a lot of traction\revenue with an opportunity to become very big. In your case maybe having a team of people also "too late for YC", and you can become successful on your own.
I also heard that they don't like video made of pieces, who knows what some alumni might not like when he sees your application.
but hey, make a first million, have a successful exit - next time apply with a billion dollars idea - they'll take you seriously!!