You could make the argument that they're technically different things, but I think in practice they are the same. If you're looking for something specific, then yeah search is gonna be more effective than hashtags. But if you're just looking for something to read, you could search for "fediverse" or trawl the #fediverse hashtag. Not everybody chooses to add hashtags to their posts so full text search is gonna find more posts than just a hashtag search (assuming full text search also returns matches on hashtag).