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How could they possibly know that their codes would be broken?

I think the blob says how they could have known:

Had the Japanese done a study of the lessons learned from this battle, they may have realized the following.

American carriers had a nasty habit of being where they weren’t expected to be

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What could they have done about it?

If my adversary has broken my codes then I'd avoid orchestrating a large military operation that relies on my adversary not knowing it is a trap.

It is worth noting that the Japanese were not actually aware that carriers were in the South Pacific until the Battle of Coral Sea. They initially assumed that land-based aircraft were attacking them in a prior operation.

I can hardly fault them for thinking that maybe, just maybe, the carriers of the US were engaging in their own operations and it was just happenstance that the two forces happened to meet. But it's the job of military planners to assume the worst, so the Japanese definitely failed to be good analysts in that regard.