Hmmm, I don't think that's necessarily true. Often times once I have witnessed a bug, I have found it just by reading through the code with the behaviour of the bug in mind.

For LLMs, this is likely to be disproportionately effective as well: especially because they don't really build up a persistent view of the codebase, they're generally re-reading it each session, and they tend to be surprisingly good at predicting the behaviour of code.

(That said, knowing where and how to gather more evidence to make things clearer is a pretty core skill in troubleshooting, so it's generally good advice anyhow)