I'm perfectly aware of what the difference is, thank you. The function you gave to ChatGPT explicitly requests a stream to format integers as decimals, separated by newlines. The version it gave as being 'unit-testable' did not, and hence wasn't a 'factually correct' answer. In some cases that may be perfectly fine, but in others it most definitely isn't.
See here what a unit test is versus integration test: https://www.testim.io/blog/unit-test-vs-integration-test/
And also see my reply to the other guy where I definitively show that not only is what chatgpt produced correct, but better:
https://news.ycombinator.com/item?id=36064097