Its Custis, not Curtis.
Not only that, but the writer and editors are under the same delusion that many others are regarding George and Martha Washington. Because of the laws at that time, the General really did not have access to her fortune from her first husband; it was ear-marked for her surviving children, and actually her grandchildren (since she outlived all of her children). The slaves and landholdings that she brought into the marriage were considered Custis property, and were handed down through that line.
Washington, however, was very wealthy in terms of Southerners of his time, based on land holdings, business ventures, etc.; and his slaveholdings, while large, were not extraoridarily so.