Apart from all the hand-waving and generalizations, this guy doesn't sound like he's ever really a webapp under serious load.
I'm not a Ruby guy, so I don't know how things fly with that, but I've never once had a problem with a webapp being slow because of the language it was written in. Bad database queries, unnecessary assets being loaded, definitely. But CPU-bound limitations caused purely because of the cost of interpretation of a scripting language? Not so much...