The planet is indeed well outside of the habitable zone, but that massively thick atmosphere is keeping the planet warm. Strip away the atmosphere, and the water would freeze and you'd have just another iceball planet. This planet is not terraformable (you'll note it does not say "candidate for terraforming").
It's a "terrestrial" world, not a terraformable world. "Terrestrial" in this sense simply means "has a solid/liquid surface", as opposed to a gas giant which does not.
Of course, describing it as a "water world" is still not technically scientifically accurate, if we use the ED art department's definition of "water world" as being "a planet covered in liquid water". If water goes above the critical point of temperature (647 K) and/or pressure (218 atmospheres), it forms a phase which is neither liquid nor gas but a "supercritical fluid", which sort of has the properties of both. So while it might have an "ocean", the physical boundary between "ocean" and "air" would not exist.