For questions like this it is important to consider that imperialism is not something that a country can just do to another. Rather it is a phenomenon that has emerged from the womb a dying explicitly colonial epoch. The remnant tendrils of explicit colonialism in the global south were the foundation of the imperialistic arrangement we live in today where the USA exploited the utter devastation of WW2 to plant itself as the sole hegemon. Considering how USSR and China were devastated by WW2, it is hard to imagine that they would come out of it strongarming other countries into doing their bidding. They were on the back foot then and still are. They don't hold sway in any country's internal matters enough to be called imperial powers.

Another way of assessing that I like is considering how Prabhat and Utsa Patnaik (Indian Marxist economists) talk about imperialism in their two books. If you read Lenin's Imperialism, it almost comes off as a checklist. Radlibs in bad faith try to map this checklist onto Russia and China of today to claim they are imperialist. For example, if China builds a factory somewhere in Africa they say "China is exporting capital" and is therefore imperialist. The checklist is useful but it shouldn't draw attention from the most important thing, which is the consequences of the actions rather than the actions themselves. Patnaiks' conceptualisation is useful for this because they identify that imperialist actions and policies exist to drive down wages in the global south. For example, when IMF structural adjustments erode public services, working masses are driven to destitution and wages go down, making the target country profitable for "foreign investment". If you accept this definition, even if Russia and China take advantage of the preexisting balance of power to trade at favourable prices, they do not actively intervene to drive down wages and costs. Because of this I don't think China and Russia are imperialist.

Don't fall for the performances of shithead liberals who pretend to care about "imperialism" while ignoring the global north and south divide and pretend that everyone does a little bit of imperialism as a treat.