Put in simple terms. They have opted to create a hybrid system. Lots of stuff working in the background on their servers. But on the most basic connections, it's still P2P. The idea would be to reduce info lost between server, player and player. They used this system for D1, but the server dealt with less info than it will for D2. This, in theory, should reduce most major lag issues, but it's not going to cure it TL;DR half dedicated server, half P2P= hybrid