You're discounting the very people whose ancient text Christianity has adopted.
From my link, again:
The book does not bear the least evidence of having been written by the prophet or even during his time; and its age must be gathered from different indications. It has long since been held that it is one of the latest books of the Hebrew canon. This is proved in the first place by the language, as considered lexically, grammatically, and stylistically (comp. on this point the commentaries, and books like S. R. Driver's "Introduction"). Only Esther, Chronicles, and Daniel are of later date. Again, the way in which Nineveh is referred to shows that the city had long since vanished from the face of the earth and had faded into legend (comp. iii. 3). The King of Nineveh, also (iii. 6), could have been referred to only in a late myth; and the legendary atmosphere of the whole story, from beginning to end, is in accord with the length of time that had elapsed since the events recounted took place. This becomes evident both in the episode of the fish which swallows a man and then casts him up alive after three days, and in that of the plant which in one night grows high enough to overshadow Jonah. These things might, it is true, be considered as divine miracles; but such an explanation can not be offered for the three days' time that it takes to pass through Nineveh (iii. 3), nor for the fasting, sackcloth, and penitent cries of the animals (iii. 7 et seq.), much less for the conception that an Israelitish prophet could preach penitence to the city of Nineveh, and that the king and the citizens would listen to him. Everything about the story is, and was intended to be, miraculous and legendary.
Your link isn't from a Jewish source, from what I can tell. That's what I was asking for.
