This doesn't explain the dramatic decrease in the civic comprehension of Americans in the last fifteen years. There was a time, I think, that Trump's lying would have been disqualifying (along with many other issues with his conduct). There was a time when Americans could keep two thoughts in their head at once. Heck, people do that in non-political settings all the time, and then act like fucking morons when it comes time to ponder the future of the country.
So what changed? Why does the electorate suck so much more now? I think it points to the centrality of the conspiratorial element -- which you expressly point to, and I would argue that it should be even more centered in our public discourse. The wide availability and general acceptance of a conspiratorial mindset is both such an important part of what ails us, and what connects our experience to the rest of the world, where right-wingers are on the rise. And that's because social media puts out an infinite quantity of that bullshit.
I don't know if it's "secret knowledge" or just permission to let loose the old prejudices that had in the past been squashed by at least something of a commitment to truth and reality, post-Watergate especially. It's probably both, in varying degrees for different people. But it's really significant.