as65p, on 01 Nov 2017 - 10:02, said:
That contradicts what he said in that interview, though.
F1R: So when did you decide [to retire]?
NR: On the grid in Abu Dhabi, before the race.
Could it be that Nico meant that as a conditional retire i.e assumed he was going to win the championship. I mean it was highly unlikely that he wouldn't have given he didn't have to win the race but finish 2nd noting the car superiority. So unless he had a reliability issue or a horrendous start, it wasn't likely he'd not finish 2nd. I also guess that when he was lined up on the grid he didn't expect that Lewis would actually try and back him into the pack, because apparently before the race the team talked about that very situation and made it clear to Lewis it wasn't ok. Not that it made any difference to Lewis.
Of course I'm speculating about all of this, but we do know is that he has said that he would have kept going had he not won the championship.