> […] why the building does not have high quality NBN connections?

Because their place of residence is connected to an embedded network that has eschewed the NBN Co and chosen to connect to a private fibre operator who sits outside the NBN. They probably also pay more compared to NBN for the same speed.

Not every embedded network supplies fibre, but some do, and that appears to be the case in their situation.

By the way, NBN has recently upgraded the network to 2Gbps, with 10 Gbps having been trialled but no availability date set as of yet.