The quote isn't talking about a Romney victory at all. If Silver's pollster weights are correct, then they will be negatively correlated with the pollsters' state-level absolute errors -- that is, more weight will correspond to smaller absolute errors. And since each swing state that a pollster surveys provides a datum, there will be quite a bit of data with which to estimate the correctness of Silver's weight scheme. In other words, if Silver's weighting is wrong, we'll know by next week.

(The partisan tilt thing is a distraction -- it's easy to correct so-called "house effects", leaving only whatever systematic bias affects the polls as a group.)