Just because people were illiterate or had low literacy levels doesn't mean they didn't engage in theory though. This is also why vanguardism was so important to historical revolutions.

But you only need one comrade who can read for a classroom sized group of people (or larger) to be able to learn and discuss theory.

Part of what Mao did so successfully was to convey complex ideas in theory in very simple to understand language. In fact, he was so effective in doing this that most of his writings manage to cross a vast linguistic gap in a very nimble way.

Theory isn't the be-all and end-all, of course. But a party or the masses without theory is like a ship without a rudder.