@Terry53029 It doesn't really help to quote IRS "Absent application of a particular treaty provision," because I think there is a treaty provision that applies.  What bothers me is that no one else has asked about this, anywhere.  There are some references online to this Polish social security system dating back only to 1999, and requiring 25 years of work to qualify (or at least 25 years of being eligible, as in this case).  So maybe that's why it has not been asked before.  I'm just not willing to go out on a limb and say the treaty applies, because then hundreds of practitioners for the next couple decades might rely on my view.