Testing is often very subtle. If you don't understand changes you made (or really didn't make because the LLM did them for you), you don't know how they can subtly break other functionality that also depends on it. Even before LLM's, this was a problem for juniors, as they would change some code, it would build, it would work on their feature, but it would break something else which was seemingly unrelated. Only if you understand what your code changes actually "touch", you know what to (manually or automatically) test.

This is of course especially significant in codebases that do not have strict typing (or any typing at all).