His argument feels like a straw man. You can and should fill in a few tests after you confirm something in the REPL. Maybe Clojure folks don't do that, but Ruby and Elixir devs often do. I would imagine he's familiar with that approach.
(I have no idea if an equivalent workflow exists in Elxir, but I've never seen it been done that way: just like people use ipython, they use iex in my experience.)
Maybe I'm just misunderstanding what people in Clojure are doing. Please feel to point me in the right direction.
The usual workflow was something like
(comment
;; experiments go here
(defn add [a b]
(+ a b))
(= 2 (add 1 1))
(= 3 (add 1 2))
(= 99 (add 9 90))
)