Well, Lanemu (which is L2) and Yggdrasil use different approaches. In Lanemu, NAT traversal is built in from the start: Starting with version 0.14, RUDP with UDP hole punching is used as the default transport protocol, allowing two nodes behind NAT to connect directly to each other something Yggdrasil does not support on its own, since routing in Yggdrasil is carried out via peering channels that must be configured manually. Lanemu also has a lower barrier to entry: you install it, launch it, and get an IP address, it’s similar to using Hamachi or ZeroTier, whereas Yggdrasil typically requires manual configuration of a peer list as soon as you go beyond your local network segment.
Google didn't kill XMPP, it died on its own. XMPP still lacks encryption by default and proper file transfers (there are 3 implementations of file transfers and all of them suck). The problem is that XMPP never had a normal protocol, and as a result, clients were forced to implement the features themselves through extensions which were not supported by all clients and servers. So it's hard to blame Google for starting to do their own implementation of features. Matrix did everything better, but for some reason people don't use it. They don't, because there's Telegram, Discord and so on.
Don't defend XMPP. It's obsolete. If you want a federation, use Matrix.
Yes, but Lanemu was designed to be a simpler solution. Ordinary users are unlikely to be able to figure out what’s mentioned above.