https://www.marxists.org/reference/archive/mao/selected-works/volume-2/mswv2_10.htm

Really good writing

While this is the thesis of third worldism/settlers it's completely untrue unless you think class struggle is completely non-existent in imperialist countries. The strength and position of the monopoly capitalists gives them the power to wage war totally against the lower classes of their own country. Third worldism almost in a sense sees the equation backwards where the reality is that revolution is stifled in the imperialist countries because those countries monopoly capitalists have such intense power they have both the carrot and the stick to prevent the class struggle from being able to escalate. The Russian Revolution was the first successful socialist revolution out of all countries because it was the weakest, most backwards, and internally factionalist imperialist power.

In this case, Mao was specifically talking about how the Japanese communists were also waging war against the Japanese imperialists class fighting for the working class. There were opportunist and trotskyite figures in the united front against Japanese imperialism that were claiming that they must abandon nationalism in favor of internationalism and sit behind the KMT, while Mao and similar forces were correctly advocating for taking up the national cause and fighting on the Frontline everywhere there was one.