How is a researcher supposed to answer that question in a way you'd accept? With a product, they have monthly sales, which they can compare to prior months, or that month in a prior year, or run them in one city but not another, etc. An election is a one shot and you're done event, and our votes are secret, so even if you could isolate a population who had seen them, you can't tie those ads to a vote. You might tie it to VOTING, and see if ads in an area depressed turnout, and researchers could do focus groups during the election, but who did the focus groups for the Russian ads? Who'd pay that bill?
Etc. Bottom line is we can't know the impact. So what do we do as a country? Say, well, we can't know the answer so we should assume it's roughly 0.0% and say, "F it. Who cares?" Or say we can't know the answer, and the completely obvious approach as a country is to try like hell to prevent it from happening again, because the risk is it did or WILL in the future change an election or many elections? I vote the latter.