We don't have a fixed money supply in the modern system when banks can issue debt. Not even close. I suggest you read the Bank Of England's paper called "Money Making in the Modern Economy" which I mentioned.
Yes, I'm quite familiar with that paper.
I'm not arguing that we have a fixed money supply. I was saying that if we were on a gold standard, in an alternative universe hypothetical, where the money supply was close to fixed, we would probably see worse price volatility.