Isn't it because a hack was involved?
Would it be considered a hack if somebody within the IRS gave those documents to the press? What if the source was a foreign entity working through an intermediary?
That is to say, what distinguishes a source vs a hack is a function of the press' investigation into the authenticity of the materials.
I'm not sure blocking the NYPost is the right call here, much as I hate to say it.
"Twitter blocked users from sharing an article that indicated Hunter Biden introduced his father Joe to a Ukranian businessman — charging that “hacked materials” were used in the story."
Very, very bad move by Twitter and Facebook. The partisan rule-bending is clear.