So it is P2P in open mode? Who hosts an instance? And how big is the instance? System wide?
The matchmaking and BGS and lots and lots of things are client-server.
But all the "realtime" stuff (seeing other human ships flying around) is P2P; whether in Open or PG.
Standard matchmaking instances are small; I believe 16 players. This number includes people joined via multi-crew. There are various tricks to increase the instance size, but it gets progressively less stable / performant. Think about it, in a true P2P network, everybody needs to connect with everybody else which means the number of connections goes up quadratically. Just 32 players means nearly 500 connections.
Nobody "hosts" a P2P session; every node is a peer. I don't know if FDev implemented this, but P2P networks also allow for mitigating against cheating - every node would check actions taken, and only allow "legal" actions. If one node cheats, the others would simply outvote it and ignore the action, or possibly even automatically kick it out.
You can get around the P2P limitations by making one player the host, effectively turning it into a mini-client-server model, but the trick then becomes choosing a suitable host with sufficient bandwidth; you also need host-migration protocols in case the host disconnects suddenly and various other bits and bobs. All up it's a fair bit more complicated. Of course, you also need to trust the host not to do anything nefarious. While it's possible to ensure that the hosts' code hasn't been tampered with, it's much more difficult to ensure that there are no network packet modifications going on unless every packet is encrypted and/or signed. Even then, the host can disadvantage nodes by simply delaying packets.
The final solution is to host each instance centrally, which alleviates most issues, but this approach is very expensive as you need a lot of very performant network infrastructure as well as sufficient servers to handle the instances. Doable for subscription-based games, not really affordable for games which are based on box-sales.