Insofar as that is true, it is to his credit for sure, not at all to the credit of slavery (on that I presume we agree at least to a point), but I think there is an important difference between learning something simply in spite of slavery (as in, not promoted, provided or facilitated, even discouraged or made difficult, but not forbidden) and learning something actually forbidden and dangerous, which literacy was in many cases. I am not convinced that you are appreciating that difference, and not at all convinced that the promoters of the curriculum do. You have, after all, at least stated a "despite" that does not appear to be in the curriculum, and it certainly could have easily enough.