Ilios is a new & unique scripting language to Visionaire Studio language that Simon is still working on. It's based on C# & the visual scripting system that he's working on is one of the reasons why he decided to create a custom scripting language.
I don't know whether or not you need to learn that over Lua script, as that will depend on what your intentions are in regards to scripting...
Do you just want to access & manipulate the data structure tables & premade functions &/or write some basic workflow functions yourself? If the answer to that is yes, then Lua script is fine, but if you need to write much more complicated scripts than that - still technically possible with Lua script, as I've written plenty of table based puzzles with it & various other things including a dynamic word parser for text input & simulated DOS, etc.- then you might want to consider looking into ilios, but documentation & tutorials for that is currently limited as it's an original scripting language that's exclusive to VS & thus not that many people have started looking into how to use it yet - myself, included as I've not found a reason to use it over Lua script, yet.