They most likely use GPON so the optic is going to see return traffic for your neighbors. So they make it hard (but not impossible) to bring your own optic or media converter.
Also the authentication might rely on weak secrets. I know my ISP provided FTTH router has a six letter password and a guessable username (derived from my last name), and I can't change either.
Though the research is quite old now. Couldn't find anything recent specifically for DT.