You are quite right - but any institution can only be judged by the standards and expectations of its time, and none of todays zoos (or any other institutions) could be judged by future standards many generations hence. This footage was taken in 1975 and I would guess that there is not much film of small zoos anywhere, taken 45 years ago. Some major well-respected parks today were little better at the time, worse even - and we are all very grateful they they did not go too...
By comparison - Wellingborough Zoo in 1965.
This piece of film shows just how popular zoos were at the time, among visitors who had never heard of the word 'enrichment'. A Polar Bear shows around 18 seconds in. I remember Wellingborough's Polars, housed in a circus training ring security fence (not a full one as it was much smaller diameter), troweled concrete floor, no roof, a tiny pool much smaller than Thorney's and a pile of white bread loaves for food.
As I said, my parents cannot have rated either Thorney or Wellingborough as I can only remember one visit to each, but both were very popular and provided what was expected by the public at the time.