Given most buildings are upwards of 90% air by volume, that's still quite dense.

Wanted to argue with you but apparently concrete is only 2.5g/cm3 so not that much different from water.

There have been ships made of concrete which I always found interesting. Ferrocement (I believe) is a form of boatcraft that can be done in your backyard without any special tools.

When supercooled they’re a bit more dense than that. But as the sibling comment said, most 30 story buildings aren’t filled with water.

the supercooling system and its nitrate gasses will freeze the core of the space ship

Presumably they mean compared to hydrolox like Delta IV or the Space Shuttle.