No, each user is a “peer”, in Lemmy they access content through the instances.
They are talking something more like IPFS.
I don’t really know much about IPFS, but I think a downside to peer to peer is the potential for content to disappear because someone turns off their computer or quits whatever application. I can’t be the only person to have a torrent stall at 80% because the rest isn’t available in the network.