Powell had cancer, which compromised his immune system. And while its true no vaccine makes you 100% immune, it prepares your immune system to target and destroy the virus quickly (in this case, before it destroys your lungs). So yeah, weak immune system makes for a weak immune response, however Powell would still be alive today if the people around him bothered to get vaccinated or wear masks.
Make no mistake it wasn't covid that killed Powell. Ignorant conservatives killed Powell, with covid, at some kind of fundraiser I assume.