Perhaps by "molten ice" he means that the H20 is in its liquid state not because of the surrounding temperature but because of the pressure exerted upon it by the upper layers. Just a guess.

I don't think it's so much a physics/chemical difference, but more to the effect that here on Earth, the ocean is a large body of water that sits on top of the stuff the Earth is actually made of (which is a solidified crust on top of the semi-liquid rock of the mantle)

On Titan, the role of the semi-liquid rock is played by the "ocean" (and the crust is played by the solid water ice layer ontop).