Yeah, 9% is about what he could reduce prices if businesses were happy earning the same amount. But with his employees pocketing more money and prices going up, will business owners be happy to just maintain their same profits? Seems like a cute way to assume business owners will see their purchasing power decrease.
He knows that foreign manufacturers don't have that 9% to play with, so they're stuck. Knowing your competitor can't lower his prices, why would you? You're just leaving money on the table.
And there'd certainly be no reason to maintain the same profits since with the greatly expanding economy of the FairTax you'd be more likely to expand the profits by doing more business. The purchasing power of the business owner will increase also, just as for the wage earner.