Ask them if they think what gear the car is in makes a difference on to the overal power results of a dyno.

Then ask if they think the diff ratio makes a difference to rwhp.

Then ask them to show you the mathematical formula that demonstrates these relationship

Power = torque * rpm is all there is to it.

Where is tyre size in this equation, where is gear ratio in this equation.

Get them to give you a different mathematical equation for power - and when they do, quickly take that equation down to the patent office so that later you can collect your nobel prize.

It's as simple as the mathematics - if they can refute your formula P=T*rpm/5252 then they have an argument - otherwise direct them to their arsehole and their elbow and see if they can tell the difference!