I guess you could claim, that in the tiny Instagram food snob bubble, heavily aged kajiki is famous for being an invention of @sushikimurakoji and anybody else who does it is an imitator. The fact that Shirayama-san is serving it must undoubtedly be related to its popularity.
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This piece was aged 65 days. Just step back and think about that. More than two months, longer than most butchers are willing to age steaks for fear that they will spoil. This has the texture and scent of any other fatty white fish. The only giveaway that this had been aged were the deep umami flavors resulting from the breakdown of complex proteins over time. Parmesan, shiitake, red wine - basically the same amino acids you taste in any complex protein that has broken down over time. It's like a bite of pure flavor. Extraordinary.