Imperialism is definitely a thing and it indeed means conquering other land. It's not orthogonal to leftism at all. Lenin literally called it the "highest stage of capitalism".
The USSR practiced “imperialism” by definition, forced deportations, annexations of territories.
Again, “imperialism” is just conquering territory + negative valence. It’s not actually a thing itself, it’s some other actual thing + It feels bad/it’s against my interests. There are lots of words used in political discourse that are like this. It’s just juvenile nonsense.
https://en.wikipedia.org/wiki/Imperialism#Definition