Your article doesn't actually address this topic (an atmosphere on Europa) at all. The reason is: Europa has no atmosphere. You can always see the stars on the surface of Europa, even in daylight. Mars's atmosphere is only 1% of Earth's, but at least it looks like an atmosphere from the surface, and provides some protection from radiation. Also, your floating cities need to be made of something that's not water. They need metals. Mars has metals; we cannot get metals from Europa unless we can get below the ice crust and down to the bottom, but that's going to be way more difficult than building habitats on the surface of Mars or Europa.