> What is the basis for this? Shell script is a scripting language (more precisely, a set of scripting languages with similar features.) The difference between shell scripting languages and other scripting languages is that the former are optimized around the need to scale down to a convenient line-by-line way to work with the system in a REPL; while the others may support work in a REPL it is not what they are optimized for.

Disagree. The defining characteristic of shell scripting languages is that they are a shell that can be scripted. 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.

So the only way a non-shell scripting language can become that powerful is to become a shell scripting language.

The primitives in shell scripts are the primitives of the operating system. The fundamental building blocks of your language are strings and files and processes, which makes it really convenient to work with strings and files and processes, AKA the operating system.