We've been over this umpteen times now. The second changes with gravitational potential, the horizontal metre doesn't. We all know that the "coordinate" speed of light varies in a gravitational field. In the room you're in. Hence it's the speed of light varying in a gravitational field. Hence one NIST optical clock goes slower than the other. When the light goes slower the second is bigger. But the metre is the distance travelled by light in vacuum in 1⁄299,792,458th of a second. The slower light and the bigger second cancel each other out, leaving the metre unchanged.