conflation between communism and USSR
dictatorship.
It's a simplification, but historically warranted, in the sense that the USSR was created explicitly with the end-goal of leading to a communist society as described by Marx/Engels in the "Communist Manifesto", i.e., the ideal society where the "contradictions of capitalism" have disappeared. Moreover, the 10 points that the "Communist Manifesto" says (Chapter II. Proletarians and Communists) are required to bring about communism were all carried out in the Soviet Union and its satellites (and brought about not communism, but Stalin, Mao and Pol Pot, not to mention an inability to feed the population).
If you want to be more precise in your terminology, I suggest the following distinctions:
Marxism is a theory. Typically 3 things are theorised. (1) How capitalism (re)produced inequality.
(2) A theory of revolution as intrinsic part of the transition from contemporary society to communism via socialism. (3)
A theory of post-revolutionary society.
Socialism. This describes post-revolutionary, but pre-communist societies, in particular USSR and its satellites.
Communism. The ideal society, post-revolution, where the "contradictions of capitalism" have disappeared.