I just counted everything including stdlib and whatnot (all that comes in the checkout). "Rubinius is still implemented more in Ruby than any other implementation out there." well that's probably true, but not really useful. If all ruby implementations had 0 lines of Ruby and Rubinius 1 would you call it "mostly written in ruby"?

10X on LOC count is bogus, just because it's a single number. It does depend on what you do, it might end up being that, being more or being less, essentially [citation needed].

Anyway, even assuming this odd coefficient, I wouldn't say 1/3 "not Ruby" is "pretty good", although "pretty good" is not a precisely defined term, so we're free to agree to disagree on that. Given the same assumptions you can say CPython is written mostly in Python (10x Python-vs-C, including all the library code).