Usually (always?) the way FiOS works is that every subscriber gets his own ONT. That ONT can be configured to output network bits on either an RJ45 port or using MoCA.
For many people MoCA is more convenient because most houses and apartments are already wired for "cable", ie RG-6 for television purposes. FiOS is offered as a "triple play" service: internet, TV, and phone. Cable is the TV part of that.
There usually isn't an existing Cat 6 connection from the ONT to the living area of the "flat". (Except we don't have "flats" here in the USA). So Verizon will use MoCA to send bits to a more conveniently located wireless access point that they can also supply.
When Verizon installed my FiOS I didn't want to deal with MoCA or their access point, so I had them run Ethernet from the ONT to my den.
I'm not clear on your objection to an ONT. They way GPON works is that the ONT has to be somewhere nearby; if it's not in your flat it still has to be elsewhere in the apartment building.
