Mathematicians and physicists try to give words to concepts, they don't always map well. As Wikipedia puts it:

Topological solitons arise with ease when creating the crystalline semiconductors used in modern electronics, and in that context their effects are almost always deleterious. For this reason such crystal transitions are called topological defects. However, this mostly solid-state terminology distracts from the rich and intriguing mathematical properties of such boundary regions. Thus for most non-solid-state contexts the more positive and mathematically rich phrase "topological soliton" is preferable.

[1]: https://en.wikipedia.org/wiki/Topological_defect

[2]: https://physics.stackexchange.com/questions/285731/what-is-a...

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 , a disruption in what is otherwise an orderly, regular structure.