Comment by the_af

I think the line of thought behind the article is making the tests be like a "living spec". Well written tests (especially those using things like QuickCheck, aka "property testing") will cover more than simply a few edge cases. I don't think many developers know how to write good test cases like this, though, so it becomes a perilous proposition.

Do note TFA doesn't suggest replacing all other forms of documentation with just tests.