Yes, you use the debugger in prod. I've read many articles by Common Lisp users talking about how great that feature is: they can quickly get production back up and running, then go to the code and implement the same fix they just did on live prod.

And it's not editing files on the server, it's actually reaching into the running code and tweaking its values.

That, I think, is the difference here. In many languages, the debugger can pause on the exception and let you inspect the code. But in every other language I've used, once you edit the code to fix the bug, you can't resume from where the debugger paused. You have to recompile the code and resume from the top. In CL, you can resume from exactly the state you were in when the debugger paused, only this time with the correct data in place. (Or even with a code fix having been applied, live, to the code).