I had the impression from the original article that they planned to put their "city" at the base of the icy crust and the top of the liquid ocean - a minimum of 10 km deep and perhaps as much as 100km. At these depths, even on a Moon with modest gravity (~0.1 g), hydrostatic pressure would be considerable.

With current technology we cannot build a pressure-resistant sphere more than ~ 100m in diameter, so it would need some internal pressure to hold the walls out - hence the bone calcium problem.

If the base were only a few hundred metres deep, it would, of course, have neither pressure nor radiation problems, but it would be damn cold outside and the trick would be to stop the city melting its way down through the icy crust due to waste heat escaping.