Just a reminder: as mentioned many times in this forum, eddy current braked dynos will not show power differences with drivetain weight/drag changes. This includes Mustang dynos and DynoDynamics dynos. Only inertial dynos measure that.
Lemme take a quick guess as to what kind of dyno that was measured on...<!-- google_ad_section_end -->
Loss of drivetrain weight doesn't really add power. Your engine is putting out the same amount as before. This is what an eddy current braked dyno is measuring.
However, when you lose drivetrain weight (or more precisely, drag), your car will accelerate more quickly. An inertial braked dyno is measuring this accelerative force (you're basically turning a big heavy drum).
So does a pulley add power? No, not really. However, does it change your rate of acceleration in the same way adding power does? Yes, definitely.
Lighter wheels/tires have the same issue/effect. They allow you to accelerate faster, but do not add to your engine horsepower.
Your dyno operator should have explained that to you, and he did a disservice to you by not. Dynos are GREAT tools, but they're only that: tools. How they're used and how the information is presented to you really determines what you, as a customer, gets out of it.
I hope this helps!<!-- google_ad_section_end -->