I think TDD purists seem to see no point in unit tests that are not written before the code. That is, if the unit tests were not used to drive the design and implementation of the code under test, they are useless. I find a lot of value in the test driven approach, but this purist view does not make sense to me. I sometimes, or often, write the code first (being able to do so in a repl in languages that support that is even better), when I find it easier to think through the problem in terms of implementation rather than expected output. Sometimes I will then comment out the code and begin writing tests and re-writing the code to pass them. I think that's what he should have done here; he could have benefited from playing around in the repl, but reduced his fear of refactoring later on by having the tests.
As an idea, maybe we could do... both? Explore at the REPL, and then commit to tests and clean up once you've solidified your approach (i.e. how most Clojurists likely operate).