They can see them. But I suspect you want to know about range, not sight.
Effectively, C3 only works between two units on the ground map.
The units on the abstract aerospace map don't have a range to the ground map. Only aerospace units with a flight path on the ground map have a range to targets on the ground map.
An air unit on the abstract aerospace map doesn't have a range to a tank on the ground map. Then it uses an air-to-ground missile. That missile assigns a range band to the attack, the unit still doesn't have a range. It can't pass along it's air-to-ground missile range band because it's only for that attack, not the unit itself. Nor can a ground unit gives it range to the aerospace unit, as the aerospace unit doesn't use ground ranges, it has an attack that attack assigns a range band.
So that's where the "Effectively, C3 only works between two units on the ground map" comes from.