Two problems with your statement. First, it suggest a sequential extermination. The Reds started killing folks as soon as the Nazis did. Second, it understates the number killed by the Communists. It should read 'millions' rather than 'thousands'.
There were about 5.5 to 6 million Polish Civilians killed in World War II. About half were Jewish, most of whom were killed by the Germans. Of the remaining 2.5 - 3 million, about half were killed by the Russians, and half by the Germans, as I recall, although the half and half split might include military folks as well.
And, yes, "thousands" does understate the murders of Poles by the Communists. In November, last year, I visited the new museum in Warsaw dedicated to the Warsaw Uprising of 1944, a moving experience. The murder of Polish patriots by the Red Army and political police kept on long after the official end of WWII as did Polish resistance against Communist domination.
My basic point was that while the Baltic republics were victims of the Hitler-Stalin pact, the Poles were its first victims.