It looks like there was a jetty for the iron ore rather than a dock, with no cranes for unloading slabs and connected to the plant with a conveyor system. Not really set up for anything other than bulk ore.
Comparing docks in the area, Portbury has an approx 40m width dock entrance, Newport 30m, Port Talbot something around 15m, Alexander Dock Cardiff 25m, Barry 25m. So that might determine what port is used.
Looking at recent ships in Portbury, there was a bulk carrier that left this week which had a width of 32m, so wouldn’t have been able to use any other ports in the Bristol Channel. No idea if this was used for steel traffic (had come from Spain) but an example of the sort of thing that would make Portbury a chosen destination.