Problem is not about common figure of speech. Truth is that Jesus got it wrongly, or Jonah's account was later addition for which writers were seeking endorsement through the mouth of Jesus.
This is similar to Jesus referring to Noah's day. In Noah's days violence plummeted; yet Jesus was not aware of this fact because he said: "As it was in the days of Noah, so it will be at the coming of the Son of Man. 38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark." (Mathew 24:37, 38)
Why such discrepancies? Richard Dwakins has got it correctly: “To be fair, much of
the Bible is not systematically
evil but just plain weird, as you would expect of a chaotically cobbled-together
anthology of disjointed documents, composed, revised, translated, distorted and
'improved' by hundreds of anonymous authors, editors and copyists, unknown to
us and mostly unknown to each other, spanning nine centuries." (God Delusion)