I don't think there's that much difference between Yamamoto, Unohana, and Aizen. Unohana openly admitted inferiority to Kid Zaraki, who had already begun suppressing himself, and that was Zaraki wielding a nameless sword. It’s honestly absurd. I doubt either Yamamoto or Aizen were anywhere near that powerful as children using unnamed blades.
Then Zaraki grew into an adult, adding the power he had suppressed since childhood, and obtained a true Zanpakutō with a name on top of that. Personally, I agree with Narita, Zaraki is straight-up superior to Yamamoto.
Some might find it odd that Toshiro is also portrayed as being that strong, but I think it aligns with Kubo's plan, he has good chance to be future head of the Gotei 13. He was already fighting a Top 3 Espada as a kid. Compare that to other outstanding Shinigami like Shunsui, at Toshiro’s age, they probably wouldn’t have been able to take on even an Espada’s Fracción since Shunsui or Byakuya, at Toshiro age were still in academy, let alone fight one of the top Espada.