Even a string length api like len(s) is not exhaustively testable; you can't prove it correct with blackbox tests.

TDD has refactoring steps which only have to preserve passing tests; refactoring can easly be the vector that introduces dead code as well as changes behavior for untested input combinations.

I suspect that a lot of code developed TDD is actually deployed on input combinations that are not covered in the TDD test suite.

A string length function developed by TDD will still work on len("supercalifragilisticexpealidocious") even though that exact string never appeared as a test case, and the consumers of that function will use it on all sorts of untested inputs all the time.