I would imagine they can get away with using much heavier, stronger and thicker materials as these things have zero requirement for buoyancy or portability (besides getting them there in the first place)
Not sure how they’d prevent it from being covered in algae and they even mentioned that they noticed sealife was using it as a home
I reckon it’d be a combination of using the hull itsself as a giant heat sync and the fact they can get some pretty ridiculous cooling from the ocean to compensate for some inefficiency with heat transfer