Aren't all RTS games peer 2 peer?
In very vague ways. In that it requires a connection to be established between a player and another party.
The difference is, Relic is pure peer 2 peer. The only server they have is one to facilitate players finding eachother, and to save stats, and I think they use Steam for all that. Everything else is up to the players.
Games like Battle For Middle Earth II, StarCraft II, Warcraft III, were all based on connections to a single host. If you couldn't connect to the host, you couldn't play. Your connections to other people didn't matter.
Think of it this way; In most games, all players connect to a single box. That box holds the game together and keeps things running. If the box fails, then the game either ends, or the players find a new box, depending on what the netcode is like. If a player can't connect to the box, then he can't play.
In Relic games, every single player is their own box. All boxes have to be connected to eachother. If a single box out of any of those boxes fails to connect to another one of the boxes, then the player gets kicked.
So instead of having 6 connections in a six player game, that are all stable, you have 30 connections all of varying stability.
As you can see, the more players you add, the bigger the problem gets. In a Relic game, if you jump for 3v3 to 4v4, you jump from 30 connections to 56.
The numbers might not be 100% correct because I didn't really take time to think about it, but that's the general gist of the difference between the standard player hosting system and the Relic p2p system.
The standard system is basically how everything works. If you ever join a Battlefield 3 server, you're using the standard system - somebody basically has a server somewhere that everybody needs to connect to in order to play.