The sad fact is that Marxist-Leninism allows a buildup of nationalist and ethnonationalist sentiment within their states, even though the goal should be to unite the workers from all socialist states against the owning class.
Literally the reason the USSR had a strong government over the Republics was to keep states like Georgia and Ukraine from becoming ethnonational fiefdoms, while sabotage and insurgency by the CIA stoked Nazism in Ukraine and all sorts of racial animosity in the Balkans, and we have seen the "fruit" of both efforts. The idea that the USSR was Russian supremacist is unhinged. Was there Russian chauvinism? Sure, but that is not the same thing.
The MLs are in the top left of the political compass, and up means racist/nationalist. And that's enough to make people think they know something about MLs.
I think this is more of a "The New York Times told me that Han Supremacy is a thing in China and that Stalin hated jews even more than Hitler did so it must be true, I'm totally a socialist though" thing lol