I'm not an economist but this makes sense to me. If new money enters a market, what's easier-- to change production or to change prices? Of course, it's easier to change prices, changing production has a high cost.
This is why eventually things end up going back to normal and the money that entered the economy is basically ineffective. If production stays the same and new prices adjust demand and supply, nothing is going to happen. So I think that money is neutral if production remains the same.