Marxist-Leninists support Soviet and Chinese imperialism for the same reason liberals support Western imperialism: they're both ideologues who want their people and ideals at the top of the global hegemonic order. They're in a war for global ideological (and possibly ethnic) supremacy, with the winner getting to rule the world.
Hate to break it to you bud but capitalism is already the world order. Now I need you to stay late tonight so we can get this shipment of bombs out to countries in Europe and the Middle East.
Of course, I should have said the Western liberals are defending their position atop the global hegemonic order, and the MLs are trying to unseat them.
But Russia and China are both capitalist countries.
They're certainly not ideologically liberal, even if they have embraced some liberal economic principles, albeit under significant state control and direction.