You can look at it this way:
- The Gold Medal is awarded to the athletes who have no other athlete before them, in this case Joseph Schooling.
- The Silver Medal is awarded to the athletes who have one athlete before them, in this case Michael Phelps, Chad le Clos, and László Cseh.
- The Bronze Medal is awarded to the athletes who have two athletes before them. In this case, there is nobody who has two athletes before him: Michael Phelps, Chad le Clos, and László Cseh all have only one athlete before them, and Li Zhuhao has four athletes before him.
- Fourth Place is awarded to the athletes who have three athletes before them. In this case, there is nobody who has three athletes before him: Li Zhuhao has four athletes before him.
- Fifth Place is awarded to the athletes who have four athletes before them, in this case Li Zhuhao.
… and so on.
So, Li Zhuhao is in Fifth Place, because there are four swimmers faster than him. There is nobody in Third Place, because there is nobody who has two swimmers faster than him.