The fact of Grant's slave ownership is widely known and the article I linked to can be easily fact checked. The ownership has never been covered up. Grant managed his father inlaw's farm starting in 1854. At some point, the slave was deeded over to him. Grant signed the paper of manumission in 1859. Some of the circumstances may be unclear, but the slave was freed by Grant.
The quotation you presented is factually incorrect. Grant did not own slaves, plural, and did not retain ownership beyond the Civil War.
