I found the moest ironic thing about this was that Jack, despite knowing himself that he was wrong, was actually right lol. In actual fact Locke didn't move the Island - Ben did.
But as far as the question goes I can't see it going beyond Jack just not willing to admit he was wrong. He can't let himself believe in anything beyond science and so when confronted with such a thing as the Island going down the plug hole then he needs to deny himself.
I don't doubt the possibility of Jack thinking something else, I just feel that it's Jack basic stubborness (as Juls commented a while ago) that prevents him from admitting the truth.
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The Enigma Heroes Is Better Than Lost
...heroes never die
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