The paper focuses on two things: default behavior and behavior with a prompt to write at least one new test.

In general — just like with humans — I find "just add more tests" to be counter-productive.

Tests make sense in a testable architecture: TDD can encourage one to be implicitly used, but it is a design, architectural choice that should be made explicit (lean to functional code; use direct, explicit dependency injection; ensure test stubs are just variants of the real implementation and fully tested using the same test as the real one...). LLMs should be prompted with this guidance instead for proper value estimation.