Yeah, plutonium makes for poor hull material too.
Less facetiously, It looks strongly like they took the density of steel (call it 8g/cm3) and roughly doubled it. It makes sense that they might use iron or steel as the base material to collapse, since it's cheap and common but still strong.
Density is nowhere near the only factor though. Titanium is valuable because it's strong but lightweight; typically around 4.5g/cm3.
It's also not a given that Superdense is even a collapsed metal. I could see Carbon being used, for example. Diamond has a density of about 3.5g/cm3, so if they can gravitationally collapse that to be 5 times the density and it's stable, that may be it. Or there may several materials that are good candidates for gravitational partial collapse.
(And just to head off anyone that was unaware, tonnes per cubic metre is the same value as grams per cubic centimetre. You can mix and match densities expressed in either measure.)