What actually is this?

Luanti is the engine that MineClonia runs on, and Mineclonia is a full featured, finished, Minecraft clone.

The Website is rather unhelpful.

Seriously. It's a little sad.

Its just a Minecraft clone?

For practical purposes, it absolutely is.

Grab Luanti for free off of F-Droid or any Linux app store, or out of the SteamDeck Desktop "Software Center". Launch "Luanti", choose "Mineclonia". (Edit: or Windows or Mac, too. - Luanti - Getting Started.

Pick a username to "Register" (local play is identical to server play) and start playing free open, wonderful, easily modded, multiplayer Minecraft clone.

If a friend wants to join, just give them your IP address - even on SteamDeck or Android phone (edit: and Windows and Mac - sorry, no iPhone, I think.)