From the Washington Post piece:

But the study doesn’t go so far as to say that Russia had no influence on people who voted for President Donald Trump.

• It doesn’t examine other social media, like the much-larger Facebook.
• Nor does it address Russian hack-and-leak operations. Another major study in 2018 by University of Pennsylvania communications professor Kathleen Hall Jamieson suggested those probably played a significant role in the 2016 race’s outcome.
• Lastly, it doesn’t suggest that foreign influence operations aren’t a threat at all.

And

“Despite these consistent findings, it would be a mistake to conclude that simply because the Russian foreign influence campaign on Twitter was not meaningfully related to individual-level attitudes that other aspects of the campaign did not have any impact on the election, or on faith in American electoral integrity,” the report states.