The same way computers can have a "bus" despite there being no large multi-passenger motor vehicle inside the laptop sitting on our desks. It's technical jargo

The same way computers can have a "bus" despite there being no large multi-passenger motor vehicle inside the laptop sitting on our desks. It's technical jargon that means something other than what the layperson thinks it means. It's probably easiest to visualize the sense in which "defect" is used in the article by the analogous concept of https://en.wikipedia.org/wiki/Crystallographic_defect https://en.wikipedia.org/wiki/Crystallographic_defect , a disruption in what is otherwise an orderly, regular structure.