You see it in the beginning of the video; simple locks. Submerge the channel horizontally to fill it, *seal the locks*... and LIFT.
Once it's sealed, it's effectively a solid object that can be moved with people pulleys & leverage, to any location & any orientation necessary to float rocks.
And let's be real...No one can argue that the Egyptians of all ancient cultures were bad at sealing things (but they may have been bad at treating their workforce like real people, who knows?) 😉😅