The reason imperialism is outlined the way it is by Marxist-Leninists is because it's a useful definition. It very specifically describes the way the world functions today, what gives rise to imperialism and why, and how to overcome it. Lenin's analysis is useful because it explains how all capitalism works towards imperialism if it is able to, and gives us the tools to overcome it. It explains why revolution happens in the global south, not the global north.
As for your definition, it's a bit loaded here. All countries meddle in others for their own benefits, the second part where you say it's against the interests of the imperialized country is doing all of the heavy lifting. Moreover, there's no systemic analysis for why this comes to be in some countries and not others. The reason Marxism-Leninism is useful is because it's actionable, and helps make predictions for the future.
As for Russia, Sudan is in the midst of a civil war right now, it's in the interests of everyone that it is stabilized as quickly as possible. Russia having an extraction industry in Sudan is not the issue at hand, nor is Russia directy contributing to civil strife. Russia has a good reputation in Africa, genuinely, for being a much better business partner than the west, similar to the PRC but not as good, and this is because Russia lacks the financial capital to dominate African countries.