So, yes, on the surface of it, this particular Ruby build might appeal to people looking for performance.
But from a language research position it is interesting to decouple concepts like GC from the language so that they can be studied and optimized as a pattern and then be potentially reapplied to any language runtime.
I agree that some concepts might be pulled into MRI Ruby 3, but OMR may be a powerful testbed for language researchers to debate GC strategies and measure effects in various languages before committing to a single language integration.