After watching the YouTube channel the Great War, I know know that the totenkopf insignia predates the Nazis. However it was adopted by the vile SS along with the double lightning bolts. I can understand while the Russians might be upset seeing their leader being portrayed as SS their own crimes against Poland and its people notwithstanding.
This is in addition to the POW's and civilians massacred at Katyn and other places.
It's hard for the Poles to see much difference between the Germans and the Soviets in WWII, except for the color of the uniforms.