Presumably the lisp-like syntax and immutable variables by default. Lua has some functional features, but it isn't a primarily functional language like this.

What is it missing though? It seems like this could be a different syntax compiled to Lua bytecode.

It could have been compiled to Lua bytecode, but I felt like having an external VM and ArkScript would have been too much. I always hate when I download a lib and 100 smaller libs come because the main lib relies on it

Small libraries are an accountability nightmare. Reducing the number of dependencies on a core library is very important.

This also seems odd to me because Lua can be embedded in a binary easily or made into a shared library since it is just C with no dependencies of its own.