They're saying if Communists do it, it's not Imperialism even if it looks exactly the same.

They are willfully committing an equivocation fallacy, using their definition of "Imperialism" as being necessarily related to Capitalism. The textbook definition of Imperialism does NOT necessarily relate to capitalism, so you are indeed in the right.

a policy of extending a country's power and influence through diplomacy or military force. -Imperialism

A non-capitalist country most certainly can do that definition. And Russian and China have both done that quite unambiguously.

So you're in the right. But you'll never win an argument against them because lies are truth.