Why would they imagine that solid metallic hydrogen might be stable or meta-stable? Diamonds are a bad analogy because diamond is a crystal. Yes, it forms under great pressure, but it's a crystal lattice with extremely strong covalent bonds linking all the carbon atoms in a three-dimensional grid.
Hydrogen becomes a metal at low enough temperature and high enough pressure, but metals form amorphous solids, and all metals melt above a temperature that is characteristic of each metal. So why would they think metallic hydrogen would not melt like all other metals, and then evaporate when the temperature goes above the boiling point?
ETA: And as for rocket fuel, given the lengths needed to go to to make metallic hydrogen it would be damned expensive rocket fuel.