You are making some good points here.

I am not an astrophysicist, but I believe nobody knows.

This is an interesting question that basically boils down to "is this form of matter stable?".

But if it is not stable in the presence of continued gravitational force, I believe "unspring" would not describe very well what would "or could" happen then. Because hypothetically the amount of potential energy stored in this is magnitudes higher than nuclear fission or nuclear fusion would provide.

If I had to guess a sample of this kind of matter would be either stable (if there is an interaction holding these neutrons together) or it would make all nukes on earth look like a firecracker.

No. That would be White Dwarfs.

A magnetar is a type of neutron star. It is made up of densely packed neutrons. It is a form of matter that is unlike anything we know here on earth. All the matter we know, all elements are made up of Atoms. An atom is neutrons and protons(+) surrounded by electrons(-). Atoms attract and repell each other.

Even in the most dense material known on earth there is 99.99999% empty space between any of the actual particles.

In a neutron star, the gravitational force has become so strong that these repelling forces of the atoms are overcome and the cores of the atoms got crushed together.

This is exotic matter unlike anything we have experience with.

Neutron stars are incredibly compact. They are around 10 kilometers across in size while having around 1.4 times the mass of our sun - which is more mass than our whole solar system has.

Despite being very different in nature Both neutron stars and white dwarfs are the remnants of former stars and both emit electromagnetic radiation (light, heat, x-rays, gamma rays) which is left over from the time they were actively undergoing fusion processes.

White dwarfs are the remnants of smaller stars, they are essentially the solid hyper-hot iron core of stars. They radiate heat and light for many millions of years before they eventually cool off. (Note that this will last longer than the universe is old now).

Neutron stars are the remnants of more massive stars. The moment the fusion fuel burns out, there is no force pushing outwards to counteract the immense gravoitational force - and in the case of these stars the gravitational force is strong enough to overcome the repelling force of protons that keeps atoms apart. The force crushes the nucleons together into a neutron soup. This crush generates immense "heat" energy which is being radiated in the form of gamma rays, radio waves and in other electromagnetic spectra until eventually these neutron stars too cool off in the distant future.

Stars that have had even more mass before the fusion reaction stopped, turn into black holes.

As far as I know there is no nuclear fusion happening in neutron stars. The parent star collapsed in a supernova because it ran out of fuel for the fusion, which was counteracting the gravitational force.

Now I'm not an astrophysicist or astronomer but aren't they describing a magnetar, which is a type of neutron star? As far as I know all neutron stars are believed to have a "solid" surface.

Solid in this case is misleading, because this is exotic matter. Neutron stars form when large stars go supernova. The collapse and the gravity are so immense, that the neutrons of the atoms that have made up the parent star actually touch.

This is why a neutron star with a 10km diameter has several masses of the sun.