I find that with a REPL in hand, I tend to play with functions while letting my mind almost drop into the problem space.
So I do less up-front design, but wind-up with somewhat better code at the end.
The other weird effect is that I find that this helps me think better about coding in something like C as well. A little experimentation at the REPL helps me understand pieces of the solution better.
He's talking specifically about writing _new_ code. I think the implication here is that Clojure encourages more consideration up-front, resulting in designs that are easier to modify down the road.
Given how much time is spent maintaining code vs writing new code this seems like a reasonable trade-off.