You've got a completely wrong idea on "soup veggies."
For one thing, the mirepoix in Luc's recipe aren't "soup veggies." Luc's recipe was for stock. Stock is meant to be strained until fairly clear (at least) and the vegetables -- as well as anything else caught in the strainer, discarded.
If you want "soup veggies," you add them when you're making soup. That is, when the stock is already completed and the first set of vegetables discarded. I know the linguistic distinction between "stock" and "soup" is blurry, but trust me on the preparation if not the semantics. A "stock" is not made to be served as is, but as an ingredient in something else. Your chicken soup may be no more than vegetables and chicken stock, but that's how you do it. The vegetables which flavored the stock have given their all already. If those vegetables were served to you, you were poorly served.
Luc's recipe was for a sort of blond (aka "light," aka "fond blanc") stock , as opposed to "fond burn" (aka "glace de poulet," aka "dark," aka "roast chicken stock"). In brown stocks, including roast chicken stock, the mirepoix or at least part of it, is slightly browned and almost always in the oven. Anytime anything is browned by sauteing, the vessel used for the saute is almost always either deglazed and the deglaze incorporated in the next step, or the vessel itself is used to cook whatever comes next. The bits of protein and sugars which attach to the pan from the heat of the saute are a cook's pure gold. I guess Luc put in the switch to get rid of the oil he used to sweat the vegetables, but am not sure.
Incidentally, the technique Luc employed was "sweating," not sauteing the vegetables. A saute is a specific process with a fair bit of heat and agitation.
Finally, if you look at twenty different recipes for making ordinary chicken stock, and throw in Escoffier, Pellaprat, Julia Child, or any other "authority," Luc's will probably be the only one which calls for sauteing the mirepoix. I'm not saying the pre-cook isn't a good idea or even a great one. Sweating the vegetables first is an old trick for intensifying their flavors, but it's not something you see in ordinary chicken stock -- where chicken is usually the predominant flavor desired. Great as a base for soup, no doubt.
Unfortunately, your hypothesis that the sweat (or even a saute) prevents oxidation in the soup, thereby causing the vegetables to retain color and texture isn't correct either. In fact, if you look back at Luc's instructions he called for cooking the vegetables for up to half an hour, until they were completely soft (presumably in order to change the molecular structure in their little interiors). You can imagine what they'd be like after a further two hours simmering. But your mistake goes back to the broader point. Stock is an "extraction." The idea is to get as much as possible from whatever is in the pot, into the stock. If you leave any flavor at all in your ingredients, you're not doing it right. Luc's method makes the extraction process really crank.
Remember, if you want "soup veggies," you get them making soup, not stock. Simply cut them in regular sized pieces and simmer them in the soup until they are done exactly as you like them.