Technically, Frederick Douglass's book is an autobiography. After all, it's the story of his life from the time of his birth to the time he wrote the book, in 1845. But it also has a lot of important omissions. For example, there's Douglass's announcement that he's gotten married, which comes totally out of the blue. Where, when, and how did he meet this woman?
That tells us that this isn't just an autobiography. Douglass has a particular political purpose in writing about his life. That purpose is ending slavery, and Douglass leaves out anything that doesn't help him do that. Little things like getting married, for example, don't have much to do with slavery, so he barely mentions it. Since Douglass has a political agenda, this piece can be considered propaganda (though not in the negative sense).
At the same time, though, Douglass's book was successful because it wasn't just a piece of propaganda. It's also a coming-of-age story. Douglass simultaneously grows up to be a man and learns to be free. His journey into adulthood parallels his struggle to find the strength and will to achieve his freedom.