Frederick Douglass makes a big deal of the fact that his narrative is true, and so do Wendell Phillips and William Lloyd Garrison in their prefaces. They all did this because they knew that people rarely argued in favor of slavery as it actually was; instead, people who were pro-slavery imagined a version of slavery where black people were happy being slaves. Slavery, according to Douglass, was synonymous with deception, so it's no surprise that people like Mr. Covey are master deceivers. But in a way, this makes Douglass's job easier: all he had to do was truthfully describe the things he had seen and experienced as a slave.
Slavery can only exist when people don't know how bad it really is. This is why slave owners tried to prevent Douglass from telling his story.
Slavery can only work when the slaves themselves are tricked into misunderstanding what it really is. This is why slave masters try to prevent slaves from being able to read or learn about freedom any other way.