That's the most overstated claim in programming. LESS is the "right tool for the job" when the job is styling web pages. Ruby is a general purpose programming language, the jobs it is good for is everything not covered by DSLs like LESS, the same as all other general purpose languages.
I question that. Ruby is very slow, dynamically typed and suffers from scaling challenges because of its multithreading support.
I think Ruby is increasingly falling into a niche: prototyping. Get a prototype up and running in a few weeks and once the proof of concept is established, switch to a statically typed language on the JVM.
Ruby was slow (still is), but that trend is changing. More and more effort is put into speeding up and modernising Ruby in general.
It might have only begun to mature, but it's definitely getting a there.
Rubinius is a lot faster than MRI and has proper multithreading support. There is also JRuby, which runs on JVM.
It's really a question of how and what you run. And yes, the JVM is one of those options.