1. It conflates the forecast with the now-cast. The former is a prediction of what will happen on Election Day (whether that day is six months away or one day away). The latter is a prediction of what would happen if the election were held that day. In theory, those two would converge to the same value on the day of the election, though in practice, that wouldn't actually happen due to computational differences.
2. Silver never says that he "should be judged" by [just] the final result. In fact, he's gone out of his way to say that. The problem is, that's the only point on the graph where we can compare a model (predicted value) to the actual (observed value). There isn't an election on any of the other days, so even if both agreed to look at the now-cast and use that as grounds to evaluate the model, we still would only have one datapoint which is nonzero in both dimensions. In other words, we have a blue line, yes, but we only have a red dot. Taleb wants to extrapolate the red dot into a horizontal blue line, and then use that to judge Silver's model, which is ridiculous.