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.
"The first victim was Poland which lost 6 million (20 percent) of its citizens (half of them Jews; half of them Christians) to the Nazis, and then thousands more to the Communists. Stalin had a personal grudge against the Poles because of their defeat of the Red Army in 1920 - in which his own stupidity had played a part"
WHoa - don't forget World War 1.5 in 1932 when Stalin stole the Breadbasket of Europe and killed 10 million of his Ukrainian enemies. more than all soldiers killed in WWI on both sides.
This certainly gave the impressionable Hitler ideas 10 years later.