While I don't want to imply disinformation will always sway an election, I think you may be drastically underestimating the number of people a disinformation campaign needs to reach.
In the US 2018 congressional races many of them were decided by an incredibly small numbers of voters, a tremendous amount were decided by less than 5% and quite a few were decided by margins of a fraction of a percent.
I'm literally in a car leaving for the airport so I can't verify the exact numbers, but I'm confident I'm close enough[1]: Kansas' 2nd district was decided by like .87%; Florida's US Senate Rick Scott won by .12%; Georgia 7th was .15%; Minnesota 1st was for sure .45%; Illinois' 13th district was something like .77%; New York's 27th district was like .28%. In 2016 presidential race, New Hampshire was decided by like 1500 votes. That's just a sample of the extremely close races across the country.
Now, while those races might seem concerning, it gets even more concerning when we consider smaller districts, county elections, and city elections where it isn't uncommon for seats to be decided by margins of 25 people or less.
The worrying thing about disinformation campaigns isn't that the disinformation is going to suddenly convert 90% of our population into propaganda vessels, it's our modern ability to target smaller and smaller demographics in key locations with information in ways that we previously were unable to do in cost effective ways. The concern (and it is a concern) is that the disinformation and often intentional outright lies will misinform just tiny tiny fraction of a percent of vulnerable logic challenged people in key areas where the ripple effects punch outside their weight.
And also of course how easy it is to swing local county and city elections, considering that a rather high ratio of local voters tend to be elderly people who may not have their skeptical hats on when the magic box tells them the opposing party is eating babies or whatever.
If you've spent any time having long casual conversations with many of our voters, you'd find out that many of these people have been convinced of and believe in truly outlandish things, they want to believe in fantastic explanations rather than what is usually a mundane explanation. We don't have to look far to find outlandish ideas floating around our population--just consider anti-vax and how it spread mostly organically, it was not a highly funded and highly targeted campaign.
Our people can make rational decisions and vote for their interests, provided they are given solid information and are not exposed to outright disinformation. It doesn't matter which direction people vote, as long as their decisions are not based on disinformation spread intentionally.
[1] I'll verify those numbers once I'm settled in, but I am confident they're within a reasonable accuracy for this topic. If you're still not convinced of the larger implications here, I'd be happy to list more. There were a whole shitload of key counties where elections were decided by margins of less than 10 people and quite a few were less than 5.