Yes, they can have tables or what have you to prop their tools up. But now we're dealing with a chicken and egg problem. How can cent-thog build said table without tools? Will the first stone age centaurs need to find elevated places to get working? I suppose they could stoop down the same way a horse would graze, assuming the connecting tissue between human and horse halves are just as flexible. But the human-side of their anatomy suddenly becomes a distadvantage. Imagine all that blood rushing to their heads as they try hit stones together or what have you. They would reaaaallly want to go out of their way to get those creative neurons going compared to say - a caveman busying its hands as it sits by the fire.