The study hasn't been peer reviewed, so shame on the authors for talking about it like it's already decided, but the skeptical part of me can't rule out that this whole thing was destined to fail on purpose.

Big Oil: "See? Hydrogen doesn't work ~~in unmodified systems~~! Shucks, guess we'll have to keep using natural gas..."

And maybe it's less nefarious, like additional proof that we can't just retrofit existing systems by changing the gas mixture, but it's at least suspicious that it benefits fossil fuel producers so neatly.