Seems pretty clear from the tweet. I've never applied to YC or don't know exactly how it works internally but it seems like they have a feedback form they give

Seems pretty clear from the tweet. I've never applied to YC or don't know exactly how it works internally but it seems like they have a feedback form they give to founders to rate the investors that come to Demo day. Based on that they give a score to each investor and they have a threshold. If an investor falls below the threshold they are not invited to demo day. I think the system is a good one.