Haven't read the whole paper yet, but below 600 Mbit/s is implied as being "Slow Internet" in the intro.
It's simply not economical enough to lay fibre and put 5G masts everywhere (5G LTE bands covers less area due to being higher frequency, and so are also limited to being deployed in areas with a higher enough density to be economically justifiable).
Most importantly, it can be heavily over-provisioned for peanuts, so your cable is future-proof, and you will never have dig the same trenches again.
Copper only makes sense if you already have it.
But telcos have colossal copper networks, and they want to milk the last dollars from it before it has to be replaced, with digging and all. Hence price segmenting, with slower "copper" plans and premium "fiber" plans, obviously no matter if the building has fiber already.
Also, passive fiber interconnects have much higher losses than copper with RJ45s. This means you want to have no more than 2-3 connectors between pieces of active equipment, including from ISP to a building. This requires more careful planning, and this is why wiring past the apartment (or even office floor or a single-family house) level is usually copper Ethernet.