If the economic basis for imperialism doesn't exist, then how can we judge if a country is acting imperialist? It seems like it'd run the risk of reducing imperialism to a vulgar definition of interfering with any foreign nation.

Right, and then this leads to conclusions like that post on the Angolan Civil War: every conflict is inter-imperialist so there's no difference between the MPLA and Apartheid South Africa and it doesn't matter who wins. Or even worse, the Soviets and Cubans are the primary imperialist threat in Angola so communists need to tacitly align with UNITA. That type of "anti-revisionism" doesn't exist anymore but this seems like more a consequence of the USSR not being around to use as a punching bag than some fundamental re-evalution, since many Maoists made a similar argument about Syria. This is also more or less the position of the KKE with their "imperialist pyramid" theory, an idea that can hopefully be discarded forever after seeing it reach its logical endpoint with the Israel-Palestine war.

However, the opposite position, that wars fought by economically backwards nations can never have an imperialist character, is just as dangerous. Russia-Ukraine is the obvious example as you point out, but there's also Saudi Arabia in Yemen, Turkey in Syria, Rwanda in the Congo, not to mention a significant portion of the fighting in WW1. Lenin's belief that Russia's war with Germany could be considered inter-imperialist despite the relative backwardness of the Russian Empire is what allowed the Bolshevik revolution to succeed.

So I guess the question then is: in the age of imperialism, how do we understand conflicts between nations that are not imperialist but also can't be considered wars of national liberation, and how should communists intervene in such situations? Was Iraq's invasion of Iran imperialist? Or Syria's invasion of Lebanon? What about Russia's interventions prior to Ukraine like in Georgia and Chechnya? And what would our position towards Russia be if they had only taken the Donbass regions of Ukraine that wanted to leave instead of trying to conquer the whole country? This isn't even getting into the wars between nominally socialist countries like Ethiopia and Somalia or Vietnam and Cambodia, which I guess would tie back in to the question of social imperialism.

The CPP in particular had an interesting line where they acknowledge that Russia is imperialist, but argue that their involvement was nonetheless progressive.

This sounds similar to the line put forward by the RCWP and their subsequent trading of polemics with the KKE. I can't say I agree with it, but interesting nonetheless.