I believe the basic idea here is that the "imperialism" of Rome is fundamentally different from modern "imperialism".

19. The “imperialism of ancient Rome,” which carried on a policy of seizure and conquest, by force of arms subduing to its hegemony a great many countries in Europe, Asia and Africa, must not be confused with modern imperialism, just as, for instance, usury capital which existed before capitalism must not be confused with usury bank capital in the epoch of imperialism. The difference between the imperialism of ancient Rome and modem imperial­ism consists in the difference of production bases. Then it was small peasant and artisan production and commercial capital; now it is enormous machine production and monopoly capital. This example, among others, shows how incorrect it is to understand by the term “modem imperialism” only a “policy,” and not a whole system of capitalist economy. The former point of view makes it impossible to understand the distinction between the imperialism of ancient Rome and modem imperialism.

• Imperialism: The Highest Stage of Capitalism, p. 125