I'm not totally sure, I'll make a couple points I personally believe.

1: nobody is immune to propaganda == everyone is equally scrupulous when confronted with propaganda.

2: a lot of people "fall" for propaganda less because it oh-so convincing and clever but because they have incentives to believe it.

3: in the digital age you do have less excuses for being really poorly informed.

4: a lot of people in the west are just straight up unreachable unless/until shit in their lives substantially changes.

5: we (actually) leftists are indeed smarter than most people, the issue is less believing that and more letting it get to our heads and making us insufferably smug.

Edit: final thought

6: I don't think being a leftist means you have to have this poetic "faith in the beauty of humanity" like some people here say you do. Plenty of past leftist (Lenin, Marx) where exactly overflowing with love for their fellow man and weren't afraid to talk shit when they felt people, either individuals or at large, were being stupid, and they were much more successful than any of us. I support communism because I think it's the right path for human civilization based on our history, not because I think ever living soul is a beacon of angelic light. You can feel cynical about your fellow man and society at large and still think the correct path for both is communism. If anything doing so will make you more adept at understanding your fellow man/society at large.