Even if this is true, it seems to me that this Trump fellow knows how to fight with people causing him problems. Going out and publicly proclaiming a "boycott" before Fox get's their announcement out, means that he can effectively argue they are lying and just pulling a "sour grapes" stunt.
He has prepared the battle-space ahead of Fox.
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Trump’s masterful, even vicious, all-out war with the media should be emulated by all our candidates.
They are a powerful and implacable foe.