It depends on the water.
Guinness is brewed with almost Pilsen-like water and the sourness is the result of the separate roast barley liquor being added back to the main wort. Roast barley steeped in water with very little alkalinity will produce a pH of around 4.5...plenty acidic. Not much need for acid malt or post ferment lactic addition.
If you brew with an alkaline water, then you are likely to need to include extra acidity in some form. Acid malt or another external acid are candidates.