Actually, his take on it isn't quite accurate. It's not "written in C++".

From http://rubini.us :

Wherever possible Rubinius is written in Ruby. Where not possible (yet), it's C++.

And further down:

The Rubinius bytecode virtual machine is written in C++, incorporating LLVM to compile bytecode to machine code at runtime. The bytecode compiler and vast majority of the core classes are written in pure Ruby.

So, it's the VM that's C++. The bytecode compiler (which is technically what's meant by "interpreter" in this case) is ruby.