The author does mention debuggers. But you don't normally run a debugger on production or a testing environment and with a REPL you can. A debugger situates a program differently than it is normally used, a REPL connects to your code/state/environment in any stage of development.

I would also not normally connect to a REPL in a production environment, but it can come in handy.

One can certainly attach a debugger to a running process in production, I've done it several times to debug gnarly issues.

I’ve done it in Elixir a handful of times to try to understand a bug that I couldn’t reproduce in my dev environment. And a few times to run some queries for a client and dump them out to CSV.

You used an elixir debugger on production?

Elixir REPL in production yeah. Not to modify anything, but to inspect the state of the world and figure out why some OTP apps were dying. I'm drawing a blank on the name of the build tool, but the "launch binary" it produces has an option to fire up a second copy of your app, keep the console alive, and connect it to the cluster as another node. Worked like a charm, although it's something I wouldn't want to do on a regular basis :)

I used the Cider debugger with Clojure with some success, but as I got better at REPL-driven development I ultimately stopped firing up the debugger

True, for Java and Kotlin I use the debugger in IntelliJ like a REPL all the time. You can even evaluate arbitrary expressions, connect to running applications even applications running on another machine.

It's not as powerful as a proper lisp repl as you cannot redefine functions or classes, but it is still very useful.