Yes, you are correct in the technical sense. However, certainly Sally being “let go” was highly unusual since the estate was in severe debt and all the other slaves were sold who were not named in the will.
https://www.monticello.org/site/house-and-gardens/sale-monticello
True, but Heming value at sale would have been low. She was 58 years old, and her sole skill was as a seamstress. Virginia law required “freed slaves” to leave the state.
By allowing her to remain technically a slave, Hemings could live out her life with her sons and not be required to leave Virginia. That was the law, how rigorously that law was enforced, I don’t know.