I'm not seeing a problem. Your drag a brush over the dirt and grit filled wheels and and then finish the rest of the car. He then went over the paint and washed away all of the dirt that was ground into the paint. It's not like he left it on there.
Should be titled... how to wash 100 cars in one day.
I watched the video after reading the title and after also writing a new how-to article on how to was a coated car, (actually any car), and do everything you can to avoid swirls and scratches.
I'd put my method up against their method any day.
I also noticed they started out with a brush that was spinning, probably from the water pressure so not a super strong rotation, and then they switched to what looked like a Montana Boar's Hair Brush. I wonder why they switched?