I knew that and probably should have stated so in my post.
But the deal was secret, and Stalin did indeed claim he was acting in defense of minorities in eastern Poland. And the various commie symp groups in the West supported him in this.
It should also be pointed out that the eastern boundaries of Poland were based not on any logical ethnic dividing line or ancient historical border but rather on the results of the Polish-Soviet War of 1820.
Acckk. Make that the “Polish-Soviet War of 1920.”