"Imperialism" isn't only thing Lenin wrote about the topic of socialism and war. Remember how Stalin says about "objective revolutionary" and "objective reactionary" and Mao about "just, progressive war"? Those are literally taken with context from the Lenin's articles written in 1914.
Ultras like to oneline Lenin and apparently ignore that it isn't WW1 anymore. Hell, Lenin did not even had any trouble in supporting one side in 1905, in purely imperialist war.