My theory is that he had to balance the journalistic scoop of the century with the risk of being arrested for illegally accessing/storing classified information. If they had noticed before he published the story then he could have been vanned and the public told that he had infiltrated a secure channel, and who would be able to say otherwise? MAGA people would cheerfully call for his execution.
Under US federal law it is generally not a crime for a person without a security clearance to receive or store classified information. The legal problems come in when they solicit it or take some other action to obtain it.
Laws matter less than they used to. When the President regularly uses the term "retribution" to describe his mode of operation, I don't blame someone for taking a more careful approach in a case like this. It shouldn't be that way, for a journalist. But a lot of things shouldn't be the way they are today.
