Oftentimes, finagling with the REPL is merely about getting the syntax right ("did I use the right number of parens?") rather than formulating assumptions. In such cases, using this tool doesn't invalidate the TDD workflow, since your assumptions don't change.

The essential complexity of writing unit tests is in formulating your assumptions ahead of time. As you correctly point out, no tool can ever solve this. However, an accidental complexity is correctly expressing your assumptions in code. Tools such as a REPL and this library can definitely help with that.