It seems that there is another reason for immodesty apart from Inadequacy. If you are walking with someone who looks at the $20 bill from a far and obviously dismisses it as fake, while you are considering going to pick it up, then you might want to think why she is doing this. It may be she is trying to convince you not to go and pick it up, so she has a shot at it later. So Deceit.
If you see a scientist, hired by the people that run the subway, walking close past the $20. When asked they may say, " I thought it is likely to be fake due to an blog post I read by Eliezer". But sub-conciously they might be factoring in the disruption to the flow of people it would cause if they bent down to investigate the $20 dollars, so the "Eliezer Post" was not their true reason. So Motivated Reasoning.
These are small scale examples of reasons to distrust the apparent and reported beliefs of others. They can also apply to society at large. Advertising and Lobbying can shift public opinion away from the truth. An example of this is sugar lobbying , So you shouldn't necessarily trust majoritarianism if people are being swayed by experts and those experts have non-truth seeking incentive structures.
Taken to the extreme this can cause paranoia. It is worth trying to look at the experts and seeing how big the incentives are for believing what they do. To take a non-real world example, if all the experts studying sugar are employed by sugar manufacturers and would lose their jobs if sugar became significantly less popular, then they have strong incentives to say sugar is not dangerous (whether through deceit or motivated reasoning). If all the experts on sugar are generic food health scientists who had tenure and could easily switch to studying some other food stuff if sugar went down in popularity, then they don't have huge incentives to say sugar is better than it is (apart from things like protecting the epistemology).
This doesn't mean you are any more right about the things you believe. You also probably have non-truth seeking incentive structures in your life. If epistemic rationality is important to you, it is probably best to try and minimise those structures you get enmeshed in.