What is the central idea of this passage? Human beings are creative when they get into trouble. Because of one man's actions, whales never eat human beings. Whales always have a sore throat because of a grating. Human beings should stay away from whales to avoid being swallowed.

Because of one man's actions, whales never eat human beings.

This is a popular way to structure a myth. The actions of one person affect the relations between both the descendants of that person and the descendants of whoever is making the "promise". In this case, the whale.

The story of "How the Whale got his tiny Throat" by Rudyard Kipling was first published in St Nicholas Magazine, in December 1897. It was collected in Just So Stories, 1902, illustrated by the author and followed by the poem "When the cabin port-holes are dark and green."

The story tells that once upon a time the Whale ate fishes of all types and sizes. At last there was only one left in the sea, a small astute fish that hid behind the whale's ear and advised him to eat a shipwrecked mariner. The Whale swallowed the mariner and the raft he was sitting on.

But then the mariner was inside, he started to jumped around so much that the Whale got hiccups and asked him to come out. The mariner answered that he would not, unless he was taken to the shore of his British home, and hopped harder than ever. So the Whale took him to the beach and the mariner came out. But in the meantime the clever mariner had made his raft into a grating which he secured in the Whale's throat with his suspenders. Forever after, the Whale could only eat the smallest of fishes.

the central idea of the passage is that:

Because of one man's actions, whales never eat human beings.