I think that Fivethirtyeight are not a reliable source anymore after how they got literally every prediction in the 2016 elections wrong.

Actually, FiveThirtyEight gave Trump the highest chances of any prediction model, so if you're going to give credence to any analytical methods, theirs was the best: https://www.buzzfeed.com/jsvine/2016-election-forecast-grades

And I think it might have changed, but I'm just not sure how many people's votes were switched by the announcement. Maybe you could make the argument that it caused lower voter turnout for the Dems, but even then I'm not super confident about that.

The only reliable data we have our polls, and polls can't really distinguish between whether people's votes actually switched or whether the turnout dynamics changed. Whether it's the former or the latter, the polls do show a very clear shift towards Trump following the Comey letter: http://election.princeton.edu/2016/12/10/the-comey-effect/

Whether this shift was caused by people changing their minds, by Republicans feeling more willing to support Trump, or by Democrats feeling less willing to support Hillary doesn't really matter. All that matters is that there was a clear shift in the polls two weeks before the election.

I think that the truth is the whole DNC anti-Bernie scandal probably hurt Clinton a lot more than the comey announcement did.

That's probably true, but Bernie and the DNC are tied up in so may other issues, and it happened so long before the election that there are too many intervening events. I mean, if the DNC didn't lean towards Clinton, would she even be the nominee? Maybe we would be sitting here arguing about how incredibly stupid it was for the Democratic Party and the Republican Party to allow themselves to be hijacked by radical outsiders, resulting in the election of an independent Michael Bloomberg as President in a 3-way election.

The thing about the Comey letter is that it was literally the last major thing to happen before the election, and it caused a clear shift in the polls - so it is easier to identify it as a discrete factor.