FWIW I am also unconvinced by the article's thesis that prediction markets "beat" polls. But I'm not sure I agree with your premise that we need to ascribe causality if markets beat polls. The "point" of markets is that they work in mysterious ways: the combination of financial incentives and the wisdom of the crowd coalesce to price things correctly. You might be able to point out why a specific market participant has a specific opinion and makes a specific bet, but doing so for the entire market is a fool's errand.
Low trust voters are hard to poll because they don't trust pollsters, they don't pick up their phone for strangers.
6 years ago this didn't matter because they were as likely to be Republican as Democratic. But that changed with Trump who appeals to these types of voters.
Then this election democrats suddenly increased their civic engagement which lead to them being much more likely to talk to pollsters. Skewing the results.