IDK if dude is smarter than you are, but he's a domain expert. He knows enough about the subject that it would likely take you something in the range of a decade of dedicated time, research, engagement, and experience to reach an equivalence. You're probably quite capable of this, if you decide to go down this path.
If you don't want to go down this path, though, know that it probably will just take that breadth of experience to develop the understanding and intuition necessary to be able to say and explain things on the subject with confidence.
You should believe it because you trust people like the one in the video when they tell you it's a good idea to clean your wounds to prevent infection. You vaguely understand this has something to do with germs, but you (probably) don't know all about the various actual diseases that cause open wound infections, their outcomes, their treatments, the history of research behind it, etc. It's an entire field of medicine, actually. Luckily for you, a lot of the important stuff can be condensed into a single important general-action for the populations: clean your wound with soap and water, then, cover it with a bandage.
So why do you trust domain experts when they tell you about first aid, and not what they tell you about hugely more complex subjects, such as vaccinations and drug trials?
Why the sudden skepticism now? During a pandemic, of all times? If this was a movie, the climax / resolution would be the development of a vaccine - thank god, the experts delivered us from this terror. What were you expecting instead? 10 years of lockdown while we wait for things to shake out?