What you have to take into account here is the state of supercritical fluids.
At a pressure above ca. 220 atmospheres and above ca. 650° K, water is in a supercritical state with some strange physics, see below link:
http://www1.lsbu.ac.uk/water/supercritical_water.html
In this state the boundary between the fluid and gaseous state has fallen and the new state is some kind of twin state of both of them and raising pressure and/or temperture makes this state only more 'liquid-like' or more 'gas-like', but you can't compress it into a liquid state.
At exactly the critical point this matter is said to even have infinite compressability (in theory).
While I am no physicist, I guess that is the state of the atmosphere above the surface on such planets.
But as you can deduce from the phase diagram, given high enough pressure and temperature, the superfluid water can be compressed into a solid state, and that's probably what the icy core consists of.
At a pressure above ca. 220 atmospheres and above ca. 650° K, water is in a supercritical state with some strange physics, see below link:
http://www1.lsbu.ac.uk/water/supercritical_water.html
In this state the boundary between the fluid and gaseous state has fallen and the new state is some kind of twin state of both of them and raising pressure and/or temperture makes this state only more 'liquid-like' or more 'gas-like', but you can't compress it into a liquid state.
At exactly the critical point this matter is said to even have infinite compressability (in theory).
While I am no physicist, I guess that is the state of the atmosphere above the surface on such planets.
But as you can deduce from the phase diagram, given high enough pressure and temperature, the superfluid water can be compressed into a solid state, and that's probably what the icy core consists of.