(Regarding the power-scaling issue: when the young King-level Austin defeated Rocks, he was with Roger and Garp; Austin merely landed the final blow by exploiting an opening.
As for Austin now, his mastery of Haki is at the Super King level—Rocks and Imu's level—though still weaker than Joy Boy. However, because his Devil Fruit has an exceptionally powerful and unusual set of mechanics, he can absolutely suppress King-level opponents. In theory, two Kings can contend with a Super King, but mechanic monsters like Imu and Austin are special cases, so theory cannot be used to judge actual combat.)
"That was really reckless of him~"
After that battle, news of Elegia's conflict spread throughout the world.
Nearly every influential faction learned that a perilous war concerning the world's safety had taken place in the Music Kingdom Elegia.
"Still... that does suit Dad's personality~"
"Don't you think so, Kuina?"
Seated atop the enormous ship's prow, the massive giant listened as the woman beside him read out the intelligence, then smiled and asked,
"Even so, Teacher was far too bold. If he were trapped inside by the Song Song Fruit forever, his life really would be in danger..."
Having received immense kindness from Austin, Kuina was clearly worried about what he had done, which was why she answered so bluntly.
Yet Sade, draped in a black cloak and dozens of times Kuina's size, merely held his straw hat and smiled.
"Don't worry~"
"A man as powerful as him could never die to a Devil Fruit."
Having grown up in a laboratory, Sade had heard more about Austin than anything else, whether from the Shichibukai or people of the underworld.
The one sentence he had heard that still made the most sense to him was,
"Never think that Austin will be troubled by