In The Narrativve of the Life of Fredrick Douglass, when Garrison writes that Douglass needed "a comparatively small amount of cultivation to make him an ornament to society and a blessing to his race," is he implying that Douglass is exceptional or that all slaves are capable of such "high attainments"?
Garrison is implying that many, if not most slaves and/or African Americans are capable of such high attainments, I think.
Frederick Douglass had the advantage of being sent to the right place at the right time to get a bit of a head start with his education. Of course, his official education was nipped in the bud, but it had been enough to give him a thirst for knowledge and his inborn intelligence and curiosity drove the rest, so he "tricked" the poorer neighborhood white children into helping him learn more (it's easy to trick those who are still young, aren't yet fully aware of the prohibition on education black children, and have not fully developed societally-induced racism yet, though).
But if Douglass was capable of it, why would not most be capable of the same thing, given the opportunity to learn and better themselves?
History has borne out the truth of his observation.