The one professional prognosticator who came closest to predicting a Trump win at 33% agrees with the letter playing an outsize role - https://fivethirtyeight.com/features/the-comey-letter-probab...
The release of the letter coincided with polls that showed that slide yet predated its release (in collection).
It's not that it didn't have an impact. Just that it probably wasn't the deciding factor.
That doesn't mean Hillary didn't make a huge strategic mistake in going after Trump's competence and character rather than focusing on the issues. She did. She totally did.