The study you linked to doesn’t support the conclusion you claimed. It provides evidence that low muscle mass is associated with cognitive decline. But it’s presented as a binary - low muscle mass vs not low. There is no talk of a “statistically inverse relationship”, as if adding more and more muscle results in less and less cognitive decline. Intuitively, it shouldn’t be too surprising that critically low muscle mass is associated with issues. Either because it causes decline in other areas or because it is correlated with it (or both). Indeed, from that paper: “Whether low muscle mass is an early marker or a causal factor of executive cognitive decline, and elucidation of mechanisms linking muscle mass to cognitive functions remain to be determined.”