If a person lives in a house with 700 rooms, that has a mortgage... do they "own" their home?
Jefferson knew the relation of that reality.
"COMMERCE BETWEEN MASTER AND SLAVE IS DESPOTISM"
He owned them. He was legally able to free them, of which he freed 10. He could sell the slaves children or their husbands or their wives. He could decide if they would be punished or not punished for attempting escape. He could decide when a slave was to be put on their own time. You may not consider that ownership, but that kind of power normally does not come by renting.