Thats amusing. You show your (lack of ) knowledge of Transit brakes quite well.
Not defending anybody that charges more time than it takes to do any job. But changing Discs on a (newish) transit is not a straight forward job and can take a lot longer than expected or anticipated. Having freed everything off and then doing it again soon after will result in a much quicker job. Anybody who has done these will know, and know why.
But I know nothing about mechanics, so you say.
So you charge how you want, but dont make stupid statements like traders don't charge by an hourly rate. Whether that rate matches an exact amount of time is irrelevant. Like charging 2 hours amount for 90 minutes, or 4 hours amount for the same isn't the point. Anybody running a (labour based) business knows an hourly cost of their business, or should do, and has to price accordingly.