It actually seems to be more like a quadruple-negative. Let's try to simplify. "No serious market observer disputes the claim that volatility would not be high

It actually seems to be more like a quadruple-negative. Let's try to simplify. "No serious market observer disputes the claim that volatility would not be higher without the liquidity provided by high frequency traders." "[serious market observers believe] that volatility would not be higher without the liquidity provided by high frequency traders." "[serious market observers believe] that volatility would [be lower] without the liquidity provided by high frequency traders." "[serious market observers believe] that volatility would [be lower] without ... high frequency traders." "[serious market observers believe] that volatility [is higher with] ... high frequency traders." "[high frequency traders increase volatility]" This, of course, is exactly the opposite of what the author proceeds to argue in the following paragraphs. My conclusion is that the author managed to create a sentence so overly complicated that even he could not understand what he was saying.