No that's not how this works
What you're describing is the Flynn Effect; A raise in IQ over time despite a decline in psychometric g. When you adjust for the FE, you see for a good hundred-hundred and fifty years intelligence has been in decline. IQ is not a measure of stored knowledge, but the two somewhat correlate afaik. It's estimated that g is like 80% heritable so maybe lead has an effect but I doubt it being that severe on an individual level before a point.
People who have become convinced they have the one true answer and seal themselves off from anyone saying otherwise (as our brains are chemically incentivized to do) gradually circlejerk themselves off of their own ideas until they become so distanced from reality they seem absolutely clinically insane.
After all, why bother getting a plane ticket to the "edge of the earth" when your friends have told you that the plane industry is ALSO in on the conspiracy? Why bother getting a job at NASA and flying to the moon to see it from space if you know NASA is ALSO in on the conspiracy and will secretly add screens to the ship that show doctored images?
They'll latch onto anything, and make up any excuse out of whole cloth, no matter how ridiculous, as long as other people also believe in it. It's logical fallacies, like most things that make humans behave bizarrely.
It's more common than you'd think, I was raised the first 18 or so years of my life to believe that everyone who believed things contrary to what our group's leaders told us were secretly and invisibly puppeted by an evil mastermind who could infiltrate our minds and plant seditious thoughts into them without us even noticing!