Is this because China hads then a predominantly rural economy so the standar suppositions about the proletariat (and lumpen-proletariat) didn't hold?
If this is right how did Mao modify the theory?
Marxism talks of proletarian revolution arising out of the class conflicts in the society and to be led by the working class. Maoism talks of a protracted peasantry revolution by eliminating the class enemies by guerrilla warfare. Maoism believes in extreme violence as the only weapon to seize power.
The key differences are in the Maoist focus on an aggraian rather than industrial society and the degree of violence to be employed in the revolution. Marx advocated strike action and other basically peaceful demonstration with escalation to more violent forms of protest if this proved uneffective. Examples of this can be seen in the labour union disputes of 1970s britain and the early days of the actual russian revolution. Maoism goes much further, advocating extreme violence and guerilla warfare of the sort the viet cong engaged in in the Vietnam war.
Perhaps it could be said that Mao was a Marxist, modifying the theory for the problems he faced in China. The most famous difference is the ascendency of the peasantry, but there's also guerrilla war and the "cultural revolution".
A large segment of the population was forcibly displaced, most notably the transfer of urban youth to rural regions during the Down to the Countryside Movement. Historical relics and artifacts were destroyed. Cultural and religious sites were ransacked.
Lots of Communists have modified or expanded upon Marxism, here and there.
But whatever his Marxist credentials, Mao was in no way a Communist as Marx understood the term, or any sensible Communist Party. Contrast the Paris Commune and the People's Republic of China.