Sometimes people just get unlucky and there's not some direct causal factor. Yes, it could have been the mouthwash. It also could have been a combination of their germline genome having some unfortunate mutations and/or a series of unlucky somatic mutations.

If mouthwash has harmful ingredients keeping it in your mouth for a prolonged duration and spitting it out instead of drinking it will not solve your problems, contrary to the quoted line.

If an patient who is an outlier exhibits an extreme habit related to the affected cancerous organ, it seems advisable to take note even if it is possible there are undiscovered more serious contributing factors.

I mean, you are making a jump from correlation to causation. There is a reason science does not use anecdotes and relies on statistics, p values, and, confidence intervals.

That being said, did you happen to know what ingredients were in play, since he was using the same product for ~>20 years it sounds like, maybe the carcinogen landscape was much more under researched that long ago? I mean leaded gas was still sold in the US 'til 96 -- even though theres just so much evidence of it being harmful to the nth degree.

And why pray tell, should I disregard cancer specialists at Sloan Kettering telling this guy's entire family not to use mouthwash?

Many available common mouthwash still contain alcohol. Alcohol's causal effect on oral cancer is rarely disputed in medical circles.

If ample clinical studies are absent, science does in fact rely on reason time to time in the interim.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6982979/

Even more reason not to use alcohol mouthwash, interesting. I did not previously use it just for the fact it dries your mouth and that exacerbates bad breath.

>And why pray tell, should I disregard cancer specialist

Not doing such a thing, your words were directly "someone I know who doesn't smoke or drink and was sub-40 years old got diagnosed with tongue cancer." -- which is different from your words here where you link a study -- and I appreciate you for doing.