I think you're running into a conflict of definitions. Usually that's what these sorts of disagreements boil down to. In order to answer whether Russia counts as "imperialist", you first have to know what counts as "imperialism".
the policy, practice, or advocacy of extending the power and dominion of a nation especially by direct territorial acquisitions or by gaining indirect control over the political or economic life of other areas
By this definition, it's difficult to say Russia is doing capital "I" imperialism, since they're not occupying and controlling a majority of Ukraine. If you count any amount of land occupied, then they are.
When you're talking to tankies, you also have to realise that Imperialism is a jargon term with a specific definition. That's explained by Lenin here, if you're interested.