Inflammation: a double-edged sword for immunity.
The fact that inflammation occurs during the immune response is a normal process. Problems usually arise when the intensity is too great or the duration is too long. That would be called excessive inflammation.
If the person affected by covid-19 has a weak immune system to begin with, the immune response will be less efficient in mounting an immune response. For the entire duration of the immune response, the inflammation continues to build up.
If the inflammation goes on for too long, then irreparable damage occurs like the scarring of lungs or death.