Decades ago (yeah, I'm old) the 9mm was known as a weak stopper because it had only ball ammunition that over penetrated, so purportedly was not a good self defence round. And this was the common thread of most gun magazines and the general culture.
So perhaps he is just parroting what he learned as a youngster from his grandfather who never heard of current bullet improvements. But either way, a 9mm must smart when you're hit with it.
