Without having a proper look it sounds like they are using VLANs to segregate the traffic.
Your ethernet outlet in your apartment will be on an access only port on a managed switch in VLAN X. When you login to the AP with your credentials it will also assign you to VLAN X. Now your wireless and LAN devices can talk while your neighbours who may well be connected tot he same hardware (WiFi access point and managed switch) will have their traffic assigned to VLAN Y etc.
Fundamentally it's pretty secure. VLANs have been around for many many years and are widely used across most enterprise and campus networks.
I'm guessing they have a centralised firewall for the complex which should keep everyone relatively secure at the expense of you not being able to port forward etc easily.
Without looking though this is just a guess