"Boil it down and this means the quantum clock loses only one second every 30 billion years, which, for navigation purposes, means a drift of a mere 1 x 10⁻⁶ degrees per hour. "

What does this mean in terms of metres off a desired position?

A nautical mile is 1852 meters

1 degree of latitude is 60 nautical miles

1 millionth of a degree is 0.1 meters

So they lose about 2.4 meters of accuracy per day or about 0.9km per year. Which is pretty good for nautical navigation purposes.