Of course, the polls themselves also acknowledge the chance of a systemic error. They're reported similar to +/- 2% 19 times out of 20. They make no claim that the 19/20 of multiple polls are not correlated.

Well, 538 model took into account historical evidence that election differences from polling averages between states are strongly correlated rather than independent; arguably, that's an effect that exists in part because such deviations are in part due to systemic errors (though the particular systemic errors may differ from cycle to cycle) in the polling, rather than pure sampling error.

But their key insight is that polling errors are often correlated, and not just statistical sampling errors that can randomly offset in either direction.

Sure, but even the worst poll-based prediction outfit isn't just aggregating poll results and using the uncertainty based on sample size to determine the probabilities of different outcomes.

Pretty much all of them are using some model derived from the past relationships of polls to election results, which will, to a greater or lesser extent, capture systemic polling error that is not unique to the current cycle in the model.