Not an MMO dev but my random guess is that there's not a lot that needs to be directly p2p. I've only played FF14 but most of the interactions you have don't require you to have precise locations of your party. Server has a tick rate and for mechanics you need to be aware of server latency. Most everything can be calculated server side and/or predicted. Many abilities often have a cast time which can be used to hide latency between clients.
Unless you are playing on console, almost nothing is p2p in gaming (Due to cheaters). The server has to simulate the entire "world", and as I understand it, the problem becomes when you have one server instance simulating a world for 100s of players.
The said, FTA I think people are focusing on the wrong side of aisle. From what I understand, I don't think anything they said is impossible; it just seems like the team is very inexperienced and MMOs are incredibly complex and given how few MMO developers there are (MMOs are popular, but they aren't diverse) they might have bit off more than can chew. I'm really not seeing how what they are trying to do is anymore complicated than Fortnite or Minecraft from a backend standpoint (I'm very green to game programming)
