It doesn't seem to be a gas, so applying pressure would be as easy as tightening the enclosure, like fastening a bolt. Cables could be made by just sticking a superconducting core inside a compression sleeve, although they'd probably have to be segmented, and breaking the sleeve in any place could make the core stop superconducting and overheat pretty fast. Other than that, it should be perfectly safe.

The values suggested are north of the ultimate tensile strength for carbon nanotubes and graphene. It’s in the ballpark of the center of the earth. I don’t think it’s as simple as tightening a bolt.

I think it’s literally tightening a few bolts : https://en.m.wikipedia.org/wiki/Diamond_anvil_cell

A DAC works because you use it on a tiny surface area (measured in microns). The application was power transmission cables.

Hell, I'd even call it "not incomparable to the pressure at the center of the Sun". I mean, it's still like ~100x smaller than the smallest estimates, but it's also only 100x less pressure than is plausibly at the center of the Sun.

"Cables could be made by just sticking a superconducting core inside a compression sleeve"

You make it sound so simple.

I expect a prototype by Monday :P

It's safe. They put the material between two diamonds, the pressure is very high but the size is tiny. https://en.wikipedia.org/wiki/Diamond_anvil_cell

What's risky about heating hydrogen, magnesium and lithium and squeezing them hard together? Apart from any addition of oxygen blowing out the building?

At that pressure, any crack in the containment would have a blast radius. Even before the subsequent thermobaric explosion.

Depends on how much it compresses and can expand when breached. The blast radius from uncompressible liquids like water is close to zero no matter the pressure, a metal shouldn't be much different.