It would be impossible to quantify Carter’s assertions with absolute certainty, but he’s probably right. It would be idiotic to claim the thousands of phony social media sites managed by the Russians had no effect on voters’ impressions of Hillary Clinton, and didn’t tilt some toward Trump. Other illegitimate factors played into his victory too, including Trump’s paying hush money (which was likely illegal) shortly before the election, to women he’d had affairs with. If Trump had won by a landslide, none of this would matter, but he won by extremely slender margins, about 30,000 votes total, in three key states. Without Russian attempts to influence the electorate in Trump’s favor, he would almost certainly have lost.
Of course, he did lose the popular vote.
More evidence: White House resistance to establishing any safeguards against Russian interference in the 2020 election. If it worked before, why fix it?