So on the one hand you had Twitter, where the impression you would have had in the first few days of November is that Trump was probably going to win the election.

On the other hand you had most other platforms like Reddit, with relatively heavy-handed moderation, where the impression you would have had in the first few days of November is that Trump was probably going to lose the election.

So when you want to make a prior judgement on an extremely consequential outcome, which a posteriori was not even close, and one information ecosystem gives you the right answer, and most of the other information ecosystems give you the wrong answer, which information ecosystems do you classify as "echo chambers"?

It's possible that this was just a fluke, but it should certainly make you update your priors on which ecosystems provide a more representative sample of base reality.