This kind of feedback is not to be taken at face value. YC says very clearly that the point of the interview is to judge the quality of the founder(s). Whatever excuse they invent to write in the rejection email is really nothing more than that: a polite excuse.
They don't feel that they can be frank and just say: "We think you're probably bad founders based on the fact you have been very slow to charge customers."
Just remember that they're attempting to judge the potential of a team of human beings in 10 minutes. Realize how fundamentally flawed (and demeaning) that concept is. It's quite possible, and even likely, that their interview selection process is worse than random chance.
Of course, they think they're good at picking. But this belief is based on the theory that the companies that they don't pick will succeed even without YC's help i.e. that there would be an embarrassing anti-portfolio.
Since many of YC's most successful founders acknowledge that they wouldn't have succeeded without YC, that theory is obviously bunk.
IMHO the YC application and selection process is reasonably good. It's something approaching a crowdsourced process. The interview is them injecting their egos into the process, to the detriment of themselves and founders.
The problem is perpetuated because the people that do luck their way in are then immediately convinced that the system works. After all, it did select them, it must be pretty darn good. This is the destructive power of ego.
Someone could beat YC at selection simply by copying them and removing the interview part of the process (i.e. just accept the top N applications). Crowdsourcing is going to most closely approximate the customer point of view, and that's what ultimately matters for startups.
It's okay that YC is kind of bad at their core function. They're still good enough to stay in business and it's their prerogative. It's just a shame that they're not improving and that there isn't anything better, yet.