I have heard the same kind of claims about Ruby, Python, JavaScript, non-typed languages, functional languages etc. as well. I still have seen zero evidence that any of those claims are true. I think those claims are often made when somebody feel super productive using language/technology X, and then jump to conclusions about X based on that feeling. Seriously, if Lisp was indeed 10x more productive than competing languages, I would have switched to it a long time ago. It would be silly not to!