From the paper that is the source of that image:
"Occupying the coastal part of the Yangtze River alluvial plain, Shanghai is topographically flat, with
exception of some hills to its west. Altitude is mainly about 3–5 m above the sea level. However,
the
average tidal amplitude of the estuary system ranges from 2.4 to 4.6 m [39] and typhoons can essentially
enhance the tide level. On 18 August 1997, the
highest tide record of 5.99 m occurred in Shanghai.
The combination of the flat alluvial plain, the subtropical monsoon climate, the tidal amplitude,
typhoons, and upstream discharges causes serious flood risk to Shanghai and threats to its sustainable
development [36,37]. The situation is exacerbated by land subsidence and sea level rising [39,40].
Shanghai has constructed massive seawalls and river levees since 1950 to reduce flood risk caused by
over-flowing of rivers and seawaters. It has built pumping stations and dredged river-based agricultural
drainage system to prevent agricultural waterlogging. Meanwhile, it has also upgraded the urban
drainage system to prevent urban waterlogging by dredging river-based drainage systems, installing
underground pipes, and building pumping stations"
The red you're looking at is artificially elevated terrain incl. part of the drainage system, to hold back the sea and drain the city (which is why it doesn't follow the natural trend of elevation decline the closer you get to the ocean). You see the same thing in New Orleans. The city itself, like New Orleans, is subsiding while sea levels rise.
Let's replace your map with a larger one from the source:
View attachment 344743
Note all the straight red lines? Those are rivers / canals - high enough up that they can pump water into them so that it can then flow into the sea. That's not what you build on. Here's where GF3 is:
View attachment 344754
3 meters above sea level. Right next to a canal. Note the huge network of canals in the area to try to keep it from flooding:
View attachment 344755
If you see the area around you laced with canals like that... you're in a serious flood zone. Your safety is entirely reliant on how good the flood defenses around you are.
China's history with flood defenses are... let's say "subpar" at best.
BTW, another graphs for you from that paper whose topo image you used. Floods are on the rise:
View attachment 344756
"Second, annual flooding and the monthly floods in the three most flood-prone months, June–August, increased significantly over 1949–2009. Third, urban waterlogging had a strong increasing trend, whereas overbank flood had a decreasing trend and agricultural waterlogging had a slightly increasing trend. These findings strongly suggested that Shanghai should pay attention to the changes in flood frequencies and particularly should shift its countermeasures for efficiently reducing the occurrence of urban waterlogging. Future studies could help to investigate the driving factors for the changes in Shanghai’s floods"