I mean, that still is pretty imperialistic. It is fundamentally extending Russia's power, which is the baseline definition of Imperialism.
If extension of power is imperialism then every state or organisation of any kind is imperialist. My uncle's small business is imperialist. My neighbour's cat is imperialist. Voltaire is cumming in his grave.
The reason people are using the word "Imperialism" to refer to Putin is mostly just to paint him in a bad light. I mean fine, but it also obscures the reasons behind his actions behind just "he wants power" which is meaningless, childish, and impossible for anyone to ever negotiate with in the future, which is probably how this war is going to end.
When historians for the last 300 or so years use the word "Imperialism" they always meant the acquisition of resources. I mean you could argue that geopolitical space is a kind of resource, but it's not the same thing as when Gibbon was talking about Ancient Rome or Lenin was talking about the British Empire.