[brazzledazzle]

>What's a shell? It's a program designed to be a layer around the OS, exposing all of the capabilities of the OS to the user in a convenient form.

I think that's what they were getting at when they said:

>optimized around the need to scale down to a convenient line-by-line way to work with the system in a REPL

The only real difference is optimization for the REPL. Many of the things you might consider part of the experience aren't even shell built-ins, they're utilities maintained separately from your shell. Powershell is a good example of a shell scripting language that has a lot more going on than your traditional shell. Native object pipelines, .net libraries, etc. but the most important thing about it is that it's optimized for the REPL.