Hardly an assertion; it's simple physics. Further, I never said "I think".

For someone who rides regularly, their power output is mostly as good as it's going to get. Say I can put out an average of 150 W over a certain time period... that equates to a certain speed over a given course. If I add drag through more weight, worse aerodynamics, whatever, my power output doesn't change - I'm still producing 150 W - but I go slower.

Adding weight doesn't change your exertion level (if you're giving it 100% in a training sense anyway), it just makes you go slower. You can go slower with more weight or go faster with less - you get the same workout.

If you don't believe me or any of the others here, go pose this same question in the Road forum... they'll be sure to educate you.