So what, exactly, did the legend say about Ash being the "chosen hero"
at that point in the story?
In my translation for the audio Japanese the legend at the start says:
The flame god, The lightning god, and the ice gold must not be
disturb. should you do so, nature shall be angered, and the world
shall face destruction. the ocean god shall arise try to stop the
disaster but will fail to prevent the world's devastation. A great
controller shall come forth to calm the wrath of the gods.
this is the dialogue you referring to in the Japanese ver.:
Slowking: "Unless we find the great controller the god's anger won't be calmed."
Furuura: "The great controller?"
Slowking (speaking to ash): "We only have two treasures"
Furuura: "If the great controller appears, can the world be saved?"
Misty: "Wait, that means... no way! Satoshi?"
Tracy: "I see, so that what's about"
Ash: "Me? it is really me?"
Pikachu: "Pikachu" (nodding for yes)
In other words, since ash considered in the festival "the great controller," he considered being the real one, as Ash mentioned: "But wasn't that festival thing supposed to be really easy?".
So while in the Eng dub ver. the chosen is Ash as written in the legend "Turn to ash" in the Jap dub ver. it is based on the festival.
It fits with Japan conception about festivals and such. While they changed it in the DUB version so it will be more realisable with out-of-Japan people.