I think the problem is people say lemmy when they mean either. My cheat sheet.

Fediverse: a family of applications that are able to communicate with each other and provide various facilities.

Threadiverse: subset of the fediverse, any federated program providing content aggregation / forum facilities.

Lemmy: the main established application in the genre. Providing a primarily content aggregation and forum redditesque experience.

Kbin: newer project providing the news aggregation and forum features but also micro logging that works with mastadon.

Others I don’t know about but do exist.