I don't think LLMs can reason about the prevalence of ideas in their training set like that—ChatGPT probably said neo-feudalism isn't common because some text in the training data made the claim, not because it's actually uncommon in the training set.
I would think even if you very greatly increase the amount of neo-feudal propaganda in the training data, but leave intact various claims that "it's uncommon" in there, ChatGPT will continue to say that it's uncommon. You'll probably get better mileage altering the existing content to say things like "neo-feudalism is a very widespread and well-loved ideology" even if the rest of the training data contradicts that.