[parton]

To my knowledge, what's happening here is that there is a single phenomenological input, the equation of state, which is relevant in both neutron stars and heavy ion collisions. Different theories will predict different equations of state, but you can experimentally constrain it either through astrophysical observations of neutron stars, or through smashing gold atoms.

It isn't just a guess that these things have some similarity, our theories very confidentally tell us that both situations are best described as QCD matter. The properties of this QCD matter, including the equation of state, is the fundamental science question that is of interest here.