People lie all the time, to themselves and to each other, frequently, often spontaneously. It's the core of Trumpism.
I am not a medical professional, but I have long worked in the related field of pharmaceutical chemistry.
I have worked on the development of compounds related to fish oil, specifically those involving esters of DHA and EPA.
I took a brief tour in Google Scholar using the search terms "cognitive impairment" and tuna consumption since 2023. There is a decent number of hits, about 500. There seems to be a body of literature indicating positive associations between fish consumption and cognitive stability and resilience. This is not surprising to me, as I know that EPA (eicosopentenoic acid) is synthesized in only three known species, seaborne algae (which is the source in fish oil), black walnuts, and human (but not other mammalian) breast milk. There has been some speculation that its presence in breast milk has neurological implications that account for some of the putative intellectual development advantages associated with breast feeding.
There is some reference to mercury in these 500 hits, not a lot however.
I suspect that the main source of physiological mercury in this country is aerosols from coal fired powerplants, and that tuna consumption is less prominent. My admittedly brief foray into Google scholar suggests as much. Whether this source represents a reason for mass insanity is not for me to say, but I suspect other factors are more prominent.
All this said one of the features of antivaxxers, which includes obviously RFK Jr, Trumper, is a paranoid concern with trace thimerosal, a salicylic acid complex of mercury formerly used as a preservative in vaccines and a few other pharmaceutical preparations, but abandoned not because of any (discredited) association with mental health, in particular autism, but because of publicly inflated fear.
It's paranoia, not science.
The guy's a nut, and it's unsurprising to find paranoia about mercury poisoning to be a feature of his malevolent promotion of ignorance. I do not credit his self diagnosis. I have never actually thought highly of the guy, whenever I've suffered exposure to him, and I believe his fame is entirely tied to his family name - which doesn't mean much to me even if it does to many other people - and not to any accomplishments, intellectual or moral or otherwise. From my perspective, he's always been a shithead, a spoiled and pampered child demonstrating few, if any, redeeming qualities. He has just embraced publicly one of the worst people in the world. I find that unsurprising.