Since the peasants belong to the proletariat, why does the political book say that "the poor peasants are the most reliable allies of the proletariat"?

Because the peasants are highly revolutionary, throughout the modern history of China, it was always the peasants who revolted first, and it was not until modern industrial civilization that a society led by the working class emerged. Therefore, these two groups can be very compatible, but the farmer is not the proletariat, because he still has land, and the workers' means of production are with the capitalists. Anyway, farmers still have land