From the article:
"I've had Claude Code write an entire unit/integration test suite in a few hours (300+ tests) for a fairly complex internal tool"
Did you catch what the author didn't mention? Are the tests any good? Are they even tests? I'm playing with Opus now (best entertainment for a coder), it is excellent at writing fake code and fabricating results. It wrote me a test that validates an extremely complex utility, and the test passed!
What was the test? Call utility with invalid parameters and check that there is an error.