I see, wikipedia is not good enough for you. No problem, I happen to read other things than wikipedia. This brochure of Lenin is very interesting. And it makes very clear why you think the USSR is not imperialist. I do agree that Lenine’s USSR was not imperialist. However neither USSR disappeared with Lenine, nor did the communist thought.

There are also implications of their analysis for our understanding of the Cold War and its place in the last half-century of history. Panitch and Gindin (2012: 12), rightly in my view, did not regard USSR as a capitalist state, but rather as a different form of exploiting class society. The USSR was imperialist in the classic, general historical sense i.e. a territorial imperialist, both internally in dominating other people, such as in the Ukraine, and externally in its post-war control over Eastern Europe. The Cold War was undoubtedly a conflict between imperialist blocks, and between different modes of production. There is no doubt about the real threat of global war that it entailed, or its terrible impact on labour movements.

https://www.workersliberty.org/blogs/paulhampton/2013/12/29/review-making-global-capitalism-leo-panitch-and-sam-gindin

Also, you seem to be implying that the roman empire is not imperialist which is not what Lenine had in mind