Yeah it’s important that we, as Marxists, therefore proceeding scientific,ally, make very clear from the onset as to what we mean when we use the term ‘imperialist’ with this more specific, narrow, Leninist definition which only really applies to modern capitalism, or more precisely the modern capitalist world-system. Conceptual clarification is essential for any scientific endeavor, including Marxism.
Even on this definition however, we can note that it is perfectly possible (and concretely, empirically, historically confirm this possibility by looking at the international situation pre-WW1) that there be several powers or polarized groups of powers each of which behaves imperialistically in the Leninist sense. The difference today is that we currently still have a more or less unipolar as opposed to multipolar imperialist (Leninist sense) world-system.
If someone calls Russia ‘imperialist’ in a different sense, then they might not be wrong, and saying that they are because our definition doesn’t apply isn’t relevant beyond the fact that there’s confusion over the concepts being used because people are equivocating between them, simply because we are using the same term/sound/word/signifier. If we do the latter we are engaging in a semantic debate disguised as, because confused with, a substantive debate.