I wouldn't use the Book 6 writeups as too much of an example. Mars should be atmosphere 1, not 3. And Europa is listed as not having an atmosphere. So listing Europa and Ganymede with hydrographics of 0 could as easily be a mistake as an example of anything.
Irrespective of that, there are other examples of worlds that include their ice coverage within their hydrographics. Terra comes to mind. There is also Nosea in the Darrian subsector. I am sure if I tried, I could find more examples.
Quite frankly, it wasn't until I started reading these boards that I even heard of the idea that hydrographics was purely "liquid" coverage. I have always read it to include ice caps (when present).
On a side note, in the Traveller Book (at the very least), it specifically lists the "De" requirements as "Hydrographic 0, atmosphere 2+".