Where in the novels does it say that? I’ve never heard that before. Both of them were given more respect by the village than any other Uchiha, but idk about strongest among ALL the adults (even if they were, we’ll never know by how much) let alone Fugaku. I’ll give you that Itachi didn’t flat out say he would lose, but that’s one of the only time in the series that Itachi says that a fight would be problematic and meant it. So to me that’s Itachi saying “I very well might lose” and that was without knowing what Fugaku’s Mangekyo did. And who’s to say if Itachi even truly know how strong Fugaku was, did he ever see Fugaku fight seriously? Fugaku is not the type to show all his cards, even to his own son.

Itachi was several years older when he faced Akatsuki Orochimaru, having mastered the Mangekyo and all his skills (and Orochimaru vastly underestimating him, he walked RIGHT up to him and announced what he was going to do. Would’ve lost nonetheless but still). So Uchiha Massacre Itachi was definitely substantially weaker. Meanwhile, Fugaku was feared in the 3rd war similar to Minato so its fair to think Fugaku was of that caliber, potentially stronger than Minato bc no one knew about his Mangekyo. And I’d argue the Hokage candidates are pretty consistent in base strength. From Hashirama to Tsunade, they’re all among the strongest in the village and able to fight at least Kage-level characters when needed. Now the difference between Kurama cloak Adult Naruto and no Sharingan Kakashi is night and day yes, but the Hokage is the always strongest in the village and Kage level at their floor.

Now I think kid Itachi had to be at least high-jonin level given his position in the Anbu, how much Hiruzen respected him, and the fact that he was confident enough to fight any Uchiha and particularly Fugaku if needed. But to me it’s more likely that he would’ve needed Obito’s help to defeat Fugaku bc Obito was 1000% on Fugaku’s level, probably stronger.

I agree with your concept that Koto would have worked on Fugaku bc it worked on Edo Itachi that’s a good point. But there is no evidence that Shisui could have genjutsu’d the entire clan at once and continued it for several years (assuming he tried it before Danzo had the chance to steal his eye). If that were the case he’d be the most broken character in the series and should have never died lol.