I think the goal is to have a language that feels like it was designed for level 3/4 but allows you to break out to level 5 as an escape hatch.
Given how many configuration languages eventually end up with a half-assed level 5 (Greenspun comes to mind) it strikes me as, at least, an experiment worth performing.
Level 1 is just values in a file. The Linux kernel uses that.
Level 2 is a list of values, e.g. ini files.
Level 3 allows nesting. JSON, XML, and YAML are here.
Level 4 allows computation but limited. Dhall and Starlark are here.
Level 5 is a Turing-complete language. Python, Javascript, etc.
RCL seems to be level 5, so I'm not sure if there is really an advantage compared to Python.