Of course, ultimately the 5x is too new for any of us to know for sure. The rollout will be so limited right now, that it may well be that nobody else has even tried to put it into modem mode, so this issue has never arisen, hence why CS can claim that it has not been reported before.
Could they have pulled modem mode from the firmware on the 5x? Maybe the 5x is still in a sort of 'experimental' rollout for XGS-PON and the firmware is in a bit of a state of flux - the 'hazards' of being on a trial!
Your idea of plugging in a third party router to the fibre is a good one, if this was RFoG then it would be a complete non starter, BPI would probably kill that stone dead. XGS-PON is different, but you would probably need connection credentials which, I'm not too sure that VM would cough up.