It's of course impossible to know precisely what the grandparent meant by their comment about Delta Vega being destroyed, but they're nonetheless correct. Assuming Spock was indeed on a moon of Vulcan (which based on the information presented in the movie can't be the case, since Kirk was marooned there after the Enterprise had been warping away from Vulcan for some time, but I digress), the creation of a black hole would likely have completely vaporized any orbiting moons, and would likely have sterilized the entire star system, and perhaps even nearby star systems.

Even assuming the red matter was somehow deposited precisely in Vulcan's center of mass, and that Vulcan's center of mass was precisely at its geometric center, and that Vulcan was a perfect sphere, and that quantum fluctuations don't exist, and that no other perturbative forces exist (like the gravity from other bodies in the system), Vulcan has non-zero angular momentum (it's spinning), which means it would not collapse cleanly into a black hole as depicted in the movie. (There are still further problems with the movie's depiction of Vulcan's collapse, but they're outside the scope of this discussion.) Instead, Vulcan's collapse would form an accretion disk.

Near the event horizon, the accretion disk would be rotating at near the speed of light. Farther out, it would merely be rotating at a large fraction of the speed of light. Accretion disks are fantastically radiative, pumping out gamma rays, x-rays, neutrinos, electrons, and even bits of baryonic matter at very high energies. In fact, black hole accretion disks convert matter to energy more efficiently than nuclear fusion does. A significant fraction of Vulcan would be converted to energy, and any moon would probably be close enough to be entirely ablated away into nonexistence, assuming it wasn't ejected from its orbit outright. The neutrino flux alone would likely be sufficient to turn the entire moon molten in short order.

Except that the entire reason for the conflict with the natives was that their home tree rested on the largest and most accessible unobtanium deposit on the entire moon. If gigantic mountain sized rocks of the shit were just floating around for everyone to see, then what the heck?

Another huge problem, if I remember correctly, is that the strength of the Meissner effect on a superconductor is proportional to its surface area, while its weight in a gravitational field is proportional to its volume, and any magnetic field strong enough to lift a mountain would be lethal to life as we (and Avatar) know it.

Face it, you can't rescue the ridiculously bad script and poorly thought-out scientific mumbo jumbo with handwavy references to background material. The script and story were just plain bad. Period. Anyone that isn't capable of switching off their brain just won't enjoy the movie.