Didn't make it up. No reason to do so, although I suspect if I spent some time on HN I could find some posts about Ruby perf that don't jive with reality.
But I will say this, do you think this guy made up his post about using Ruby to try to write this game? If you don't think it fabricated, do you think he's alone?
I do think there is a fundamental problem, especially with respect to perf, that a lot of people have never looked at what code actually runs on their box. Never looked at the emitted or generated asm. Don't know the cost of calling a method, much less invoking some framework routine to do something. And then you couple this with a lot of devs think their the cats meow because they got x trans/sec and hear horror stories about the ceremony and inefficiency of C++.
And it doesn't help with the blogs and articles that constantly come out saying that Java or Ruby or Lisp is faster than C/C++ (the answer is never pleasingly straightfroward).
While I appreciate that none in your Ruby group would think this, I'd also ask why do they believe this? It's no more reassuring if they don't have a clear understanding of why C++ might be better for the job.
Lastly, I should add that the conversation was a heated discussion where I was playing devil's advocate that I thought they were going to switch off of Ruby like Twitter. I don't think they really appreciated it, although I wasn't actually being all that serious at the time.