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.

That still goes away when you close emacs/slime. I think a sketchbook would be more like an ipython notebook or a mathematica page, etc.