If we have fewer predictions, we need to be correspondingly less confident in our assessment of the quality of the predictor, to be sure. 538 has made hundreds

If we have fewer predictions, we need to be correspondingly less confident in our assessment of the quality of the predictor, to be sure. 538 has made hundreds (at least) of predictions, and across those has seemingly performed well. You can quite reasonably make a claim that most of those don't translate perfectly to the specific setting of a presidential election. But at the worst we should treat them as irrelevant and conclude that we don't know much about 538's quality. That's a far cry from a demonstration that their predictions are poor, which is what was claimed.