I think you’re confusing the “extending the power and dominion of a nation” bit and are treating that line as if it refers to two entities, as it it read “extending the power and dominating a nation”.

“The dominion of a nation” is basically its possessions.

The passage is really saying that Imperialism is “the policy, practice, or advocacy of extending the power and territory or resources of a nation especially …”, though personally I’d replace “nation” with “entity”, since I do think the capitalistic use is also appropriate. It’s really about an entity wanting to expand (or consolidate) its control.

Anyways, with that clarification, I don’t think whether or not Russia is doing an imperialism is under question.