There isn't any concrete evidence even that daily flossing actually helps.
But dentists push that, and numerous other things that weren't "rigorously tested, vetted and peer reviewed" all the time. Orthodontists devise new pieces of head gear all the time to put on people that look like torture devices, and they charge people for this privilege, despite no research, no publications and no peer review because their industry doesn't indulge in those things the way regular medicine does.
None of that is considered "grossly negligent." Yet when a guy tries to give people things to do that don't cost them money and may improve their quality of life and makes it publicly available for free, oh, NOW suddenly he needs a 44 year longitudinal study with 4000 cohorts and a multimillion dollar research budget so he can say something without getting his license ripped.
It's a case of "do as I say, not as I do" from his own industry. If he were selling a $3k piece of headgear that requires repeated visits over a decade, no one would be batting an eye because that preserves the status quo.
The idea that putting your tongue on the roof of your mouth is grossly negligent sounds like complete and utter hysteria dressed up as scientific conscientiousness, from the least scientifically conscientious discipline out there! If he was saying shit like "put a piece of steel between your teeth and bite down on it all night while you sleep", yeah, that's the kind of claim that can do harm. But simply telling you where you to put your own tongue in your own mouth on a daily basis and the loose research he has done is enough to at least start on something.
Hard to do fucking research when people are trying to steal your livelihood though.