There is some shared medium in all the PON applications, but the segregation of traffic is handled within the PON implementation. All the customer and 3rd party service provider should see would be a transparent VLAN-tagged Ethernet pipe.
I'm not a PON expert so I don't know how security is handled. I don't know what would happen if I went out and bought my own GPON modem, plugged it into the fibers,and tried to sniff my neighbor's traffic. That's sort of outside the scope of the discussion though.
I do know if you look up all those acronyms and read the wikipedia articles, you'll know more about the future of telecom than most of the people who work in telecom. :)