You're on to a good point here, I think.
Mr. Dux is certainly not alone in making outlandish and unsupported claims to support his martial arts credentials. A more relevant concern is what does he bring to the table that makes him interesting besides his own self-promotional assertions.
Helio Gracie made a lot of questionable self-aggrandizing claims during his life. (Most of them could be classified as extreme spin rather than outright lies, but still he wasn't what I would call a beacon of honesty or moral rectitude.) However he also put his reputation on the line with a number of legitimate, publicly documented challenge matches. More importantly, he helped teach and raise a generation of fighters who went on to prove the effectiveness of BJJ in a variety of contexts.
Mas Oyama made a number of claims which were either greatly exaggerated (his record of challenge matches in the U.S.) or downright fraudulent (chopping off the bull's horn). However he also created an art which has spread worldwide and repeatedly been proven effective in one-on-one unarmed combat just as BJJ has.
Bando grandmaster Dr. Maung Gyi fabricated an impressive military record and lied about it for years before being exposed. However he also helped spread an effective fighting art across the U.S., produced a decent crop of amateur full-contact fighters, and his martial expertise is praised by the Dog Brothers, who are some of the toughest martial artists around.
As for Mr. Dux, what is his legacy other than his self-promotional claims? He got a popular movie made about the story he tells. Other than that, I don't know. Is what he teaches any good? I have no idea. I've never seen it. His history may be bunk, but it's possible for an art to be effective despite a bogus background story. If I knew any Dux-ryu practitioners, I'd be happy to work out with them to see what I could learn, but I've never met any. I've never even seen the art on video or read a really good description of it.