I think the most important point that I'm not sure the article emphasises prominently enough is the purpose of both is completely different, which is why of cou

I think the most important point that I'm not sure the article emphasises prominently enough is the purpose of both is completely different, which is why of course one doesn't replace the need for the other. Testers' work is focused around the user. User input in, user outcome out. Tests pass to the extent the user has a satisfactory outcome. How that outcome was achieved with code is of no concern whatsoever. TDD is a tool for engineers to have confidence in code. Function calls in, function return values out. Tests pass when code behaves as expected. How that code is then used as part of a wider system, and the outcomes of that, is of no concern whatsoever.