Wow! A daytime star tracker! What wavelengths does it use?
It does say that higher in the atmosphere will always be better from an interference perspective (makes sense, as then there will be less atmosphere between the sensor and the stars), so that is why they market this specifically for use on aircraft. At 250k per unit it's not something you'll be able to mount on your car anytime soon.
On clear sky, you can often see Venus and Sirius with naked eye, so surely easier still with a precise instrument.
I don't think that's what they claim. They write "tracking capacity to within a few arc-seconds, equivalent to 1 meter at a distance of 70 km.", so I interpret that as a way of visualizing how small of an angle a few arc-seconds is.
This provides an absolute navigation reference, so a relative error after a certain distance traveled doesn't make sense either. The final navigation system would use inertial navigation combined with this.
For marine celestial navigation, there are fifty-eight stars that are regularly used / have been standardized on:
* https://en.wikipedia.org/wiki/List_of_stars_for_navigation
With electronics it may be possible to see more than with the naked eye ("Mark One Eyeball"), and have a larger 'almanac' with digital storage than is practical with paper.
As a side note: using only one star and the Sun would give you two lines of position, and they would potentially intersect at two locations, so if you don't have any estimate of your position you don't necessarily know where you are between the two:
* https://en.jeandusud.com/two-equations-for-celestial-navigat...
If you do have some idea, then you'd pick the intersection which is closest.