Each platform have different functions and is a different tool.
How do you imagine all the functions of PeerTube should fit inside Lemmy?
All the platforms are already unified. You can subscribe to a PeerTube channel from Mastodon. You can post to Lemmy communities from Mastodon. People on Pixelfed can follow you on Mastodon.
It’s my understanding that something is only part of the Fediverse if it can communicate with the ActivityPub protocol. Something that the Matrix protocol cannot.
Matrix is federated though, but with another protocol. Element X uses Matrix.