What they mean is it doesn't matter how much water you use as long as you end up with one gallon of concentrate gone and no more than 8,000 square feet wet. It would be very hard to apply one gallon evenly over 8,000 square feet without diluting it, so that's why they concentrate it.

According to Google, you need to apply 19 ounces of concentrate to 1,200 square feet. Put 19 ounces into the Ortho sprayer. Screw on the top and set the dial for 3 ounces per gallon. That will mix the concentrate at a rate you can control. Spray the entire lawn and if you have some left over, do it again. If you have a LOT left over, then change the rate to 6 ounces per gallon and spray again. Spray until the entire yard is covered more or less evenly.

Thanks dchall. This website is always so helpful!