I have no connection with ValvTect, however, I believe the ValvTect retail fuel supply concept has merit - the notion of guaranteed on spec, water-free fuel is hard to argue with. I've been to their facility in Chicago and the plant that makes their (and other's) additives, they seem intent on delivering a quality product and take, and investigate, customer complaints very seriously, again in my experience.
Having said that, ValvTect doesn't refine fuel, the fuel still comes from a wholesale rack supplier, one that is approved by ValvTect. ValvTect spot checks their distributors' fuel and tanks for quality and water content. When the fuel is delivered to the retailer the ValvTect cocktail is added. I've watched this process on several occasions, and photographed it. In one case, after adding about half the fuel to the storage tank, the driver of the delivery truck (not marina staff) opened a bin attached to the chassis and removed a graduated plastic measuring cup and a jug (labeled "Valvtect Marine Premium Diesel Additive with Bioguard Microbicide, the jug of gasoline additive, was labeled with a Sharpie, "gas additive") of the ValvTect additive. He poured in what I assume was the appropriate amount of additive for the diesel quantity he was delivering.
Is there an opportunity for errors to occur in this process? I would say yes. Lubricity, detergent and other retailer-specific additives are nearly always added at the "fuel rack" by the way, where fuel is dispensed into the delivery truck. You've noted no doubt that fuel being delivered to Shell, BP and Exxon gas stations isn't delivered exclusively by those refiners' own trucks (anymore), the fuel is generic and "additivised" in fuel industry parlance, with those companies' specific additives to make them unique, Shell's V-Power, Chevron with Techron etc. at the rack, and I've been told by wholesalers those additive systems sometimes fail, meaning some fuel doesn't get the additives specific to its brand. In most regions in the US there may be one pipeline or refiner that provides fuel. Where I live, virtually all truck fuel I've ever purchased for marine use comes from a refinery in Yorktown, Virginia and/or via the Colonial Pipeline (if you like this kind of stuff, the history of this pipeline, which first transported fuel in 1963, and runs from Texas to New York Harbor, passing through many major airports, is fascinating, it's actually two lines, one for gasoline and the other for jet, diesel and heating oil). Otherwise, with very few exceptions, the fuel is generic before being additivised.