I believe they just meant that tests are easy to generate for eng review and modification before actually committing to the codebase. Nothing else is a dependency on an individual test (if done correctly), so it's comparatively cheap to add or remove compared to production code.

Yup. I do read and review the tests generated by LLMs. Often the LLM tests will just be more comprehensive than my initial test, and hit edge cases that I didn’t think of (or which are tedious). For example, I’ll write a happy path test case for an API, and a single “bad path” where all of the inputs are bad. The LLM will often generate a bunch of “bad path” cases where only one field has an error. These are great red team tests, and occasionally catch serious bugs.