> He's saying that if everyone has extra money, then everyone can afford higher prices.

UBI only gives everyone extra money if it is unfunded; typical UBI proposals are funded by either increased progressive income taxes or cutting other spending (which produces income to the recipient), and so is (in first order effects) zero-sum. (Typically, it replaces rapid clawbacks from means testing with slow clawbacks by progressive tax increases.)

That’s not how market prices are generally set. Competition and demand keep them low. It might happen the way you describe sometimes but it’s far from a guarantee.

Then you can simply encourage competition to drive down prices lower.

You'd think so, but history shows different. If everyone has the same extra money, prices go up to match. It's a type of inflation I guess.