Yes, $6B comes out to roughly $16M per day, but $6B is not 2.5% of the $1.5T in income taxes that the treasury collects each year, it is less than half a percent thereof. (Still significant, of course, nonetheless.)
I believe that the claim is that Apple pays 2.5% of the corporate income taxes collected each year. That would be a subset of the total amount of income taxes collected by the treasury each year.
Yes, in either case Apple pays a significant amount of tax to the US government.