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.

Regards,Alexandr Alex@reviyou.com