I think the issue of feedback is a bit different from that of reasons why a group isn't selected. What YC seems to be saying here is more like, even if the group had done everything "right" for their project, that project/idea just doesn't seem to have the potential of another group's. (In the opinion YC).

So you could ask for feedback separately from asking why you weren't selected. The potential issue there is that groups will still interpret it that way and say things like "YC said they rejected us because of this, but that's a ridiculous reason to reject...."