Lumen is an experiment in making a Lisp (or a Lisp-like thing, depending on how finicky you are about the name) in which the universal data structure isn't a list but a hashmap (hashtable, whatever you call it). That's because hashmaps have proven to be the most convenient thing for exploratory programming. So instead of 'everything is a list, including source code', you get 'everything is a hashmap, including source code'.
We also wanted it to be self-hosted, and to compile (transpile) to Lua as well as JS, in as straightforward a way as possible. JS and Lua are very close semantically, so our approach with Lumen was to take the semantic intersection of those two and then build the simplest thing we could on top of it. As a result, it's quite different from Arc, which is semantically a classical Lisp.
Here are a couple of old comments I posted about this idea, before Scott implemented it: