The README kind of addresses this:

> if there is only a need to build a small simple application, Glimmer DSL for LibUI could be a good convenient choice due to having zero prerequisites beyond the dependencies included in the Ruby gem. Also, just like Glimmer DSL for Tk, its apps start instantly and have a small memory footprint.

And glancing at the screenshots, that look can be achieved in FLTK which is lightweight, active, and been around decades.

Correct if I'm wrong, but FLTK is not using native widgets so it is not the same thing.