They're not really suggesting a defeatist attitude so much as encouraging you to allocate your resources. You only have so much time, energy, and knowledge. To be blunt, if you're passionate about something then you need to invest the time to become knowledgeable in it.
If these two variables are not aligned then you're actively adding noise to the system and I'd argue that you are making the system less democratic. The world is fucking complex and the devil is always in the details. You may notice that the loudest voices are not the most knowledgeable voices. They will never seek details unless it is to critique their opponents and go no further. The loudest voices are not those seeking truth, they are seeking competition. Seeking truth is a cooperative game, where the main goals are aligned and there is no singular winner. It is positive sum. But the loudest voices are playing games to win, often creating competition where there may not have been before. Usually they claim a game is zero sum, when often it is not.
The point of democracy is to distribute power, and not necessarily uniformly. Our modern industrial and economic worlds runs on specialization. At the very core. This is because these processes are so complex that no individual can obtain mastery over even a significant part of the system. There are no physicists who are experts in all of physics, no chemists who have mastered both organic and physical, no climate scientists who understand all of climate, no engineers who master electrical, mechanical, aero, and civil, there are no master full stack engineers. Do you really think political affairs are any different?
For some reason we pretend we can be experts in even larger domains where experts struggle to obtain expertise in a narrow niche. If systems are chaotic, where small differences are critical, then overconfidence has disastrous consequences.