That's frankly pretty intellectually dishonest.

That wasn't a failed prophecy. That was a correct prophecy that was only deferred by repentance which in many cases is the point of God sending a prophet to call the people to repentance. I'll expand...

Jonah 3:4 is not considered a failed prophecy, despite the fact that Nineveh was not destroyed in forty days as Jonah declared, for several key reasons:

1. Conditional Nature of Prophecy

Although Jonah's prophecy explicitly states, "Yet forty days, and Nineveh shall be overthrown!" the understanding of prophetic declarations, especially in the context of the Bible, often carries an implicit conditionality. This is made clearer by God's interactions with other prophets, where warnings of judgment are often intended to prompt repentance. God's character, as revealed in Scripture, is merciful and longsuffering (Ezekiel 33:11), desiring repentance over destruction. Jonah's prophecy was a warning of what would happen if Nineveh did not repent.

Nineveh's repentance in Jonah 3:5-10 shows that the prophecy's purpose was fulfilled:
"Then God saw their works, that they turned from their evil way; and God relented from the disaster that He had said He would bring upon them, and He did not do it." (Jonah 3:10, NKJV)

The prophecy was never unconditional. God responded to the Ninevites’ change of heart, sparing them from destruction because they repented.

2. God’s Mercy

The key theological point in Jonah's story is God’s mercy and willingness to forgive those who turn from their sin. Jonah himself was upset when God spared Nineveh (Jonah 4:1-2), knowing that God is "gracious and merciful, slow to anger and abundant in lovingkindness, One who relents from doing harm." Jonah knew God’s nature to be forgiving if repentance occurred, which explains why the destruction did not take place.

3. Purpose of the Prophecy

The purpose of the prophecy was to bring about repentance, not to simply declare destruction for destruction’s sake. In this sense, the prophecy was successful because it achieved the intended result—Nineveh's repentance. The outcome was contingent upon the response of the people.

4. Biblical Precedent for Conditional Prophecy

There are other examples in the Bible where God warns of impending judgment, but relents when there is genuine repentance. For example, in Jeremiah 18:7-8, God says:
"The instant I speak concerning a nation and concerning a kingdom, to pluck up, to pull down, and to destroy it, if that nation against whom I have spoken turns from its evil, I will relent of the disaster that I thought to bring upon it."
This passage clearly illustrates the principle that God’s warnings of judgment can be averted by repentance, as was the case with Nineveh.

Conclusion

Jonah 3:4 is not a failed prophecy because it was a warning meant to bring about repentance. The prophecy was conditional, and when Nineveh repented, God in His mercy chose not to bring about the destruction. This shows the consistency of God's character as both just and merciful, and reinforces the principle that repentance can lead to divine forgiveness.

So we're back to the 100%.