PeerTube support for offloading transcoding to other machines is <a target="_blank" rel="noopener noreferrer" href="https://github.com/Chocobozzz/PeerTube/issues/947">being discussed</a>, but not yet implemented.
PeerTube uses ActivityPub because this federation protocol is recommended by the W3C and is also used by other projects like the social network <a target="_blank" rel="noopener noreferrer" href="https://joinmastodon.org/">Mastodon</a>.