No, the phase diagram is valid under all conditions. It's where you draw a curve on the diagram that determines what the conditions are (or, of course, vice-versa). One would expect that the conditions in the deep interior of a gas giant would be far removed from the "interesting" part of the diagram, where the critical point & triple point are found, and I think that's what you mean to say.

We expect that the core of Jupiter is made of "rock", perhaps surrounded by metallic hydrogen (i.e., Metallization of fluid hydrogen at 140 GPa (1.4 Mbar): implications for Jupiter). But if we go down from the top of the atmosphere, we will surely pass through the part of the phase diagram where the critical point & triple point are found. But what we see depends on the physical conditions, which will determine the curve on the phase diagram, which represents what we see on the way down. I think it is generally agreed that there will be no distinct transition from gas to liquid to solid, nor from gas to solid. But that depends on the model you use for the atmosphere, which in turn depends on what you think the atmosphere is made of, which is determined from observation. So there is some wiggle room. We do see clouds, and if there is any "surface" below them, I think it must be a liquid surface, since the light elements that are the primary constituent, will not solidify at such high temperatures unless the pressure is much higher.

See, for instance, The interior of Jupiter, a chapter in the book Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, 2004.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell