In order for the Sun to use metallic hydrogen in the production of solar heat, that metallic hydrogen must maintain its lattice form even when the temperature of the Sun reaches into the millions of degrees. How can such a process be happening?
Holmlid wrote about that in the newest paper that was linked earlier by Ahlfors. Since not everybody can access it or cannot/don't want to sci-hub it, I've also shared some notes I took from the paper (you have to expand the quoted section in my comment), which you didn't read apparently.
He writes that the inner solar temperature is thought to be about 15 million Kelvin, corresponding to an energy of 1.3 keV. The bond energy of H(0) in state s=2 is roughly similar, while that of H(0) in state s=1 is four times higher and therefore H(0, s=1) might be stable inside the Sun (for how long? not clear). At the photosphere (surface of the Sun), due to the temperature of 5800 K (~0.5 eV), higher spin states (which would be less strongly bound together) of H(0) would also exist, and I think, also the less dense Rydberg matter/H(1) form.
The superconductive nature of metallic hydrogen must be protecting its lattice structure from any particles or radiation that solar activity can produce. A positive feedback loop that makes the metallic hydrogen superconductivity stronger as the outside environment becomes more energetic must be in place.
At the temperatures involved it would not be superconductive or superfluid, as he mentions in the paper. In a work published in 2016 (open access) he observed a transition temperature at a few hundred °C (definitely above room temperature), depending on the p(0)/D(0) and carrier material, and seemingly consistent in behavior to the critical temperature (Tc) of a superconductor. The small clusters mentioned - H3(0) and H4(0) - have not been observed to lift in a static magnetic field (Meissner effect) and so they're not thought to be superconductive (but no conclusive proof exists for this state within the material, as far as I'm aware of).
By the way, since Holmlid is now basically suggesting that the Sun (and similar stars) could be composed of Rydberg matter/ultra-dense hydrogen and that related H(0) formation/destruction processes occur within it and its atmosphere with implications on its energy output and lifetime, I don't expect him restraining himself from proposing other, perhaps even more controversial ideas. This could either be a good or a bad thing.