This is for a limited subset of Ruby - almost no popular Ruby gems would run under it. It's more like PreScheme [1] (ie. a subset of a language oriented at C compilation).
I don't think these compete in the same niches right now. Full Ruby almost certainly requires a JIT.
This is what I've been wondering after only a cursory glance ("It...generates optimized C code" from the OP). Interesting that mruby itself got a major version update around the same time (in just the past few days) https://github.com/mruby/mruby/blob/master/doc/mruby4.0.md
The revenge of Rational Unified Process, Enterprise Architect and many other tools.
Instead of UML diagrams it is markdown files.