I watched the video, and was taken back by Dr. Allen’s phrasing, and I quote…

“He [Frederick Douglass] wrote a biography in which he described how the mistress of his slave owner began to teach him to read.”

“mistress of” indicates ownership, meaning the woman was also the property “of” Douglass’s slave owner. I did not look up Douglass’s exact words, but this phrasing is very intentional and accurate phrasing by Douglass, or a very deep, unconscious bias in Dr. Allen’s brain.

If this is a direct quote from Douglass, an educational scholar such as Dr. Allen should have recognized the meaning of the possessive "of".