Not being centralized has nothing to do with being trustless. The fediverse is also decentralized, yet you, me and everyone else has to log in to a specific server. If I try to login via lemmy.world, it'll fail. I have to login via programming.dev. Does that make lemmy and the fediverse trustless? No.

Even the top answer on that SO question explains that the use case of hash trees for git is different from that of blockchain