That is what she claimed on her show. I am less than 100% certain.
Counterexample: If someone left kiddie porn in her mailbox would she be 100% protected by 1A to publish it?
I am not a lawyer, but there is a dividing line somewhere between what is protected by 1A and what is not. Kiddie porn is on one side of the line. Exactly which side of the line illegally obtained tax returns fall on is not as clear cut as you suggest.
You can SAY or even DO anything you want. However, the 1st Amendment does not protect you from being prosecuted, jailed, or even killed for it.