When the Cv of a diatomic gas goes from 3/2R to 5/2R there is an intermediate part that is complicated. A photon gas has also a strange Cv. I think it's not easy to dismiss superdeterminism using just the Cv, because sometimes Cv is more complicated.
[Disclaimer: I really really dislike superdeterminism. I just think that Cv is the wrong reason to dismiss it.]
For sure. The transition from 3/2kT to 5/2kT is a good example of a frozen-out mode getting "unfrozen". Within the transition we have to count density of states and not screw around with classical analogs. ( I'm an optics person, so I think the Cv of a photon gas is normal and understandable ... If I take half an hour to write it all out again!) This is technical stuff we're talking about ... but it's not difficult for particle physicists to address!
Anyway I don't think I'm dismissing the idea of "superdeterminism". I'm only puzzled that directly measuring degrees of freedom, which was a central reason for adopting quantum mechanics, isn't even mentioned in an article where the public is being told there are fewer "real" degrees of freedom than generally thought.
For "degrees of freedom" substitute "density of energetically available states." We're taking some states off the table, we have to explain whether there are measurable consequences.
I don't understand superdeterminism and I'm not qualified to dismiss it.
[Disclaimer: I really really dislike superdeterminism. I just think that Cv is the wrong reason to dismiss it.]