I haven't seen anything about Polish Social Security.  The tax treaty says, 

3. Notwithstanding the provisions of paragraph 1, payments made by a  Contracting State
under the provisions of the social security or similar legislation of that State to a resident of
the other Contracting State or to a citizen of the United States may be taxable only in the first·
mentioned State.

And I'm not sure that's the most current version. 

https://www.congress.gov/113/cdoc/tdoc5/CDOC-113tdoc5.pdf  

You might put Poland in the subject line of another question. We used to be able to change the subject line, but that was then and this is now.