> aren't we really saying voters are not individual agents but rather a mob subject to manipulation by propaganda?

I disagree with this interpretation. It's creating a sort of false dichotomy -- voters can still be individual agents AND ALSO they can be manipulated by propaganda. And the key is that propaganda doesn't have to be wildly successful in order to impact a democratic process. It just has to convince enough people to sway an election. That is, and always has been, one of the trade-offs of democracy. That's why we say "democracy needs an informed electorate to survive" -- because an informed individual is less likely to be easily manipulated.