Their other comment elaborates on this more:

Until the link /c/books shows any user, with only one click, the aggregate of all “books” communities in a single place, without subscribing or even logging in. Then lemmy will stagnate because it is failing to live up to its promise of federated decentralization

They want a link like /c/books to work like multireddits did on reddit to collect together books-related communities for improved browsing and discovery.