The model is a tiny part of the machine -- a necessary part but of the lowest order.
Saying giselle makes them their millions back is like saying the windshield on veyron makes bugatti their millions back. It's an essential part, of course, but it's trivial. They could have designed the windshield a million different ways and no one would know the difference. Similarly, the fashion companies could have dressed up a million different models in their clothes and no one would know the difference.
To me this is totally false. What you say might have been true prior to Giselle building her brand, when she was a random model, but now, she offers instant roi opportunity for a company.
Giselle has millions of fans who adore her. A company who chooses to use her as a spokesperson, can build instant credibility with a massive audience by using her. If you're a marginal brand looking to revamp your image, or a current brand looking to tap into a huge market, using Giselle is a great vehicle to make that happen, and to drive sales across the board.
Your example might be a bad one. People do know the difference between Giselle and a model they never heard of.
Giselle is a world famous star. She's in all the mags, all over the tv. Young women love her, and when they see her wearing a brand, that brand becomes cool and they want to be part of the image that Giselle provides. They respond to that by opening their wallet.
Say I started an athletic apparel company. How many people would take that company serious and also buy my products if I had Kobe endorsing the gear vs. some random NBA player you never heard of?