When I learned and told my daughters that FDR was among those who handed this country to the communists, one of my daughters asked her high school history teacher about it. He said that's true, but if he taught that he wouldn't have a job. So some of them know and hide the truth for the sake of their job, but I'm betting most of them don't even know.
My mother used to churn butter and salting the butter is a very labor intenseive job which she did with a wooden paddle. She would work and work the salt in, until (hopefully) there were no more pockets of salt. That's what the enemy is doing with the truth. Just working the lies in until the truth becomes something else. In this case, I agree with Chant. The author probable didn't/couldn't vet everything. I accept and pass over things because of ignorance in that field. We are not in an enviable position to tease out the truth these days, which is why this forum is so helpful. If one of us doesn't know something another one might. God alone could weed out the lies these days. We have as much chance of doing that as we would of picking all the salt out of that butter. And only dedicated truth seekers even try.