By “equivalent of multi-storey apartment buildings” is meant “multi-storey apartment buildings” — the Roman empire pretty much invented those. In Rome itself there are probably some of the originals which are still in use, but with modern additions (plumbing, electricity).
Ostia was, as I recall, a major port for the Roman navy. As such it was likely populated by a lot of sailors and their families, as well as support people and merchants you'd find in any port. I wonder if the sailors might influence this finding of unusually high cases of rickets for a so-called small town rather than a city (though as I said, it isn't described as a town).