The difference here is that the code you wrote in the REPL dies with the REPL.
A REPL is more of a "scratchpad", to stay in the family of metaphors being used. Scratchpads and sketchbooks serve similar purposes, but the latter generally is used for things intended to have a bit more permanence and also to be capable of re-visiting.
In any case, I'm pretty sure the main point here is "you need to write a lot of varied code if you want to become an expert at software development".
I was thinking along the lines of SLIME, where your evaluated code is preserved by the editor. Sorry for being unclear. Been a long time since I used a raw REPL.
There's still a useful distinction to be made -- the "sketchbook" talked about is the collection of stuff you've done over some longer period of time that you can look through / look back on. It's not a description of a tool, although as you noted the language used in the last paragraph of the post is very similar to the language people use to describe the use of a REPL.
I'm a fellow emacs user, I use SLIME-like functionality practically every day. My "sketchbooks" are my ~/workspace folder and my github account. I have some older "sketchbooks" in the form of tarballs of my ~/workspace folders from old machines.
I thought that basically everyone who wrote software had similar collections of little projects, finished and un-finished, until about a year ago when it came up in discussion with a few other devs who didn't.
A REPL is more of a "scratchpad", to stay in the family of metaphors being used. Scratchpads and sketchbooks serve similar purposes, but the latter generally is used for things intended to have a bit more permanence and also to be capable of re-visiting.
In any case, I'm pretty sure the main point here is "you need to write a lot of varied code if you want to become an expert at software development".