The definition of Imperialism you are working from is not the same as those replying to you. You are working from the non-Marxist definition of imperialism while others are working from the Marxist definition. From Prolewiki:

Lenin is often credited for having synthesized a Marxist analysis of imperialism with the publishing of his pamphlet Imperialism: The Highest Stage of Capitalism in 1916, most notably on the foundation of the earlier work of John A. Hobson entitled Imperialism: A Study. Beginning with the first paragraph of his pamphlet, Vladimir Lenin wrote that rapid growth of industry and concentration of production in growing enterprises represent the key characteristic of capitalism.[1]

Multiple theorists have updated, deepened, developed or critically engaged with the classical analysis of Imperialism. Other theorists developed different conceptualisations, including most notably Kwame Nkrumah, remaining situated within the framework of scientific socialism. Most recently, the concept of neoimperialism has emerged in the work of Cheng Enfu.

The development of imperialism in the global economy also reinforces a dialectical relationship between core-periphery countries, mainly dependency and subordination of underdeveloped countries to imperialist economies. In conjunction with these developments, new theoretical models were proposed to understand developments, such as dependency theory and world-systems theory.

Lenin's Imperialism: The Highest Stage of Capitalism for more information.