George Washington's were inherited and he freed them in his will.
George Washington did not free the slaves he held in his own right in his will. He had no authority to manumit his wife's dower slaves.
The linked authority you assert for your claim does not go to the will of George Washington but to a Youtube video by Prager U which spews nonsense.
The actual will is explicitly clear:
Item Upon the decease of my wife, it is my Will and desire, that all the slaves which I hold in my own right, shall receive their freedom. To emancipate them during her life, would, tho' earnestly wished by me, be attended with such insuperable difficulties on account of their intermixture by Marriages with the Dower Negroes, as to excite the most painful sensations, if not disagreeable consequences from the latter, while both descriptions are in the occupancy or the same Proprietor; it not being in my power, under the tenure by which the Dower Negroes are held, to manumit them.
In his will Washington expressed the desire that they be freed upon the death of his wife.
Washington's slaves were freed by Martha Washington some time after Washington died.