I don't think the goal is a filter at the level your thinking about. The real question is simply is are YC rejects a better pool to consider than the random ideas they are already receiving.
On the upside, they are looking at a pool of people that got it together enough to apply to YC with 1 or 2 ideas, got rejected and still want to give it a go. That means they are a least willing to push back a setback and not simply spamming ideas looking for money. On the down side, some of the best people / ideas have been removed. Overall it's probably a better group of people and ideas than would apply to a less well known funding source.
I applied and didn't get an interview. And I agree that a filter of some sort is needed for YCReject to be meaningful. I'll persist in my efforts, but that doesn't imply I will be successful.
More to the point, though, I think much of the value of YC is in the network of alums and mentors they provide. Unless there is some buyin from those mentors (or others of their value) then I don't think there's much point to YCReject except as a team to cheer you through the low points.
On the upside, they are looking at a pool of people that got it together enough to apply to YC with 1 or 2 ideas, got rejected and still want to give it a go. That means they are a least willing to push back a setback and not simply spamming ideas looking for money. On the down side, some of the best people / ideas have been removed. Overall it's probably a better group of people and ideas than would apply to a less well known funding source.