My guess is that they were thinking of the times when Moscow (“Stalin” in ultraleftoid speak) assisted the Spanish Republicans, seized some strategic territory from profascist Finland, established a sphere of influence in Poland (after the British and French governments failed to agree to an antifascist pact with the Soviets), overthrew the anticommunist dictatorships in the Baltics, and assisted Korean communists against an anticommunist dictatorship.

Basically, imperialism is when you do stuff in somebody else’s country; exploiting it for its resources isn’t really that important.