Solomon Northup is basically pointing out that without first-hand experience, it is downright impossible to comprehend exactly what he endured over his dozen years as a slave. There is the potential that people would believe Northup exaggerated what he went through. (“White men playing con artists to a black man? Imposssible!”) Northup is just glad it’s all over, and he got through this experience alive.
In modern times, this is the equivalent of, “Unless you have actual personal experience in the matter, don’t say anything.” As for memoir techniques, this is that world-weary approach that came by purely by chance. While they are some things that people intend to happen to themselves (me leaving the Deep South for College is one of them), there are other events that are not. It makes you apply the “For Want Of A Nail”, you change a small, seemingly inconsequential event, all of sudden, the repercussions could be massive. (Hence that’s why all time travel stories by default have to use “But you can’t rewrite history! Not one line!” Approach.)