The solution here is distributed trust by proxy. You start with a single exchange between two trusted peers, and build from there. As long as every individual link within the network is trusted, then any route between two disconnected endpoints can be trusted as well. As the network grows there is a very high statistical likelihood that there will exist many individual trust graphs between two nodes, which provides redundant validation.
I have always thought this would make a cool chat app. You enter the network by scanning someone’s QR code to become their validated peer, and then you can theoretically communicate with anyone else on the network by exchanging keys via trust graphs. You could then build a social network on top of it which shows you how many hops it takes you to get to some celebrity or some shit.
Unfortunately, Lemmy demonstrates pretty clearly that decentralized systems are just as vulnerable to propaganda and brain rot.